Using a timer with GET_APPLICATION_PROPERTY

I'm using a timer (incorrectly), and in the WHEN-TIMER-EXPIRED trigger, I am getting this error:

"ORA-06502: PL/SQL: numeric or value error
Cause: An arithmetic, numeric, string, conversion, or constraint error occurred. For example, this error occurs if an attempt is made to assign the value NULL to a variable declared NOT NULL, or if an attempt is made to assign an integer larger than 99 to a variable declared NUMBER(2). "

I suspect I'm doing something wrong when I supply the name of the timer to GET_APPLICATION_PROPERTY.

The code to where I create the timer and also the WHEN-TIMER-EXPIRED code (where I suspect the trouble is) is directly below.

Thanks in advance.
-Brian

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In my WHEN-NEW-FORM-INSTANCE trigger, I initialize a timer like this:

   TimerID := find_timer ('EDITTIMER');
   if not id_null (TimerID) then
      delete_timer (TimerID);
   end if;
   TimerID := create_timer ('EDITTIMER', 1000, REPEAT);
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Then in my WHEN-TIMER-EXPIRED, I want to have some code execute when this timer fires, but I receive

declare
   CurTimer VARCHAR2(40) := Get_Application_Property ('EDITTIMER');
   x number;
begin
   if CurTimer = 'EDITTIMER' then
      x := show_alert ('test');
   end if;
end;