How to fix "Using $this when not in object context" in php error log?

I have an OJS journal running on Apache/2.4.35 (Win32) OpenSSL/1.1.0i PHP/7.2.11.

In order to fix a "non-static method PKPApplication::getRequest()" error:

PHP Deprecated:  Non-static method PKPApplication::getRequest() should not be called statically in C:\xampp\htdocs\journal\plugins\generic\customHeader\CustomHeaderPlugin.inc.php on line 128

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I ended up adding the word "statitc" in two files:

on line 237 in C:\xampp\htdocs\journal\lib\pkp\classes\core\PKPApplication.inc.php

233:    /**
234:     * Get the request implementation singleton
235:     * @return Request
236:     */
237:    public static function getRequest() {
238:       $request =& Registry::get('request', true, null); // Ref req'd
239: 
240:       if (is_null($request)) {
241:          import('classes.core.Request');
242: 
243:          // Implicitly set request by ref in the registry
244:          $request = new Request();
245:       }
246: 
247:       return $request;
248: 

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and on line 143 in C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php

138:    /**
139:     * Return PKP request object
140:     *
141:     * @return PKPRequest
142:     */
143:    public static function getRequest() {
144:       return $this->_request;
145:    }

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I now have the following error in php_error_log:

Slim Application Error:
Type: Error
Message: Using $this when not in object context
File: C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php
Line: 144

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The whole error is here:

Slim Application Error:
Type: Error
Message: Using $this when not in object context
File: C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php
Line: 144
Trace: #0 C:\xampp\htdocs\journal\lib\pkp\classes\security\authorization\internal\ApiAuthorizationMiddleware.inc.php(39): APIHandler::getRequest()
#1 C:\xampp\htdocs\journal\lib\pkp\classes\security\authorization\internal\ApiAuthorizationMiddleware.inc.php(72): ApiAuthorizationMiddleware->_authorize(Object(Slim\Http\Request))
#2 [internal function]: ApiAuthorizationMiddleware->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Slim\App))
#3 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(ApiAuthorizationMiddleware), Array)
#4 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Slim\App))
#5 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Slim\App))
#6 C:\xampp\htdocs\journal\lib\pkp\classes\security\authorization\internal\ApiCsrfMiddleware.inc.php(46): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#7 [internal function]: ApiCsrfMiddleware->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#8 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(ApiCsrfMiddleware), Array)
#9 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#10 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#11 C:\xampp\htdocs\journal\lib\pkp\classes\security\authorization\internal\ApiTokenDecodingMiddleware.inc.php(82): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#12 [internal function]: ApiTokenDecodingMiddleware->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#13 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(ApiTokenDecodingMiddleware), Array)
#14 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#15 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#16 C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php(68): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#17 [internal function]: APIHandler->{closure}(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#18 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(Closure), Array)
#19 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#20 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#21 C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php(127): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#22 [internal function]: APIHandler->{closure}(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#23 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(Closure), Array)
#24 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#25 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#26 C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php(132): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#27 [internal function]: APIHandler->{closure}(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#28 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(Closure), Array)
#29 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#30 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#31 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(117): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#32 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\App.php(392): Slim\App->callMiddlewareStack(Object(Slim\Http\Request), Object(APIResponse))
#33 C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php(124): Slim\App->process(Object(Slim\Http\Request), Object(APIResponse))
#34 [internal function]: APIHandler->{closure}(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#35 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(Closure), Array)
#36 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#37 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#38 C:\xampp\htdocs\journal\lib\pkp\classes\handler\APIHandler.inc.php(132): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#39 [internal function]: APIHandler->{closure}(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#40 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\DeferredCallable.php(57): call_user_func_array(Object(Closure), Array)
#41 [internal function]: Slim\DeferredCallable->__invoke(Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#42 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(70): call_user_func(Object(Slim\DeferredCallable), Object(Slim\Http\Request), Object(APIResponse), Object(Closure))
#43 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(117): Slim\App->Slim\{closure}(Object(Slim\Http\Request), Object(APIResponse))
#44 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\App.php(392): Slim\App->callMiddlewareStack(Object(Slim\Http\Request), Object(APIResponse))
#45 C:\xampp\htdocs\journal\lib\pkp\lib\vendor\slim\slim\Slim\App.php(297): Slim\App->process(Object(Slim\Http\Request), Object(APIResponse))
#46 C:\xampp\htdocs\journal\lib\pkp\classes\core\APIRouter.inc.php(115): Slim\App->run()
#47 C:\xampp\htdocs\journal\lib\pkp\classes\core\Dispatcher.inc.php(143): APIRouter->route(Object(Request))
#48 C:\xampp\htdocs\journal\lib\pkp\classes\core\PKPApplication.inc.php(281): Dispatcher->dispatch(Object(Request))
#49 C:\xampp\htdocs\journal\index.php(68): PKPApplication->execute()
#50 {main}
View in rendered output by enabling the "displayErrorDetails" setting.

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I understand that I probably needed to add "static" higher up in the APIHandler.inc.php file, but I am not sure where.  And maybe I don't need "static" in the PKPApplication.inc.php?  I am attaching the file referred in the error.  Could someone point me in the right direction?

APIHandler.inc.php

Many thanks in advance!