Uncaught TypeError: Cannot read properties of undefined

I keep getting an error when Im trying to find the value of a select2 selected element via:-

alert($("#txtCompetencesSuppliersAdd-Supplier").select2('data')[0]['supplierID']);

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So, I thought I could test it with typeof undefined as:-

if (typeof $("#txtCompetencesSuppliersAdd-Supplier").select2('data')[0]['supplierID'] === undefined) {
    alert("Please select a supplier first");
} else {
    alert($("#txtCompetencesSuppliersAdd-Supplier").select2('data')[0]['supplierID']);
}

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But still get the error message:-

Uncaught TypeError: Cannot read properties of undefined (reading 'supplierID')

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The alert works fine if an option is selected, I just want to show a warning if one isnt selected.

Any suggestions?