Fixed issue where Block Grid elements in areas cannot be properly filtered

[LennardF1989]

We ran into this issue while playing with a NotificationHandler to filter allowed element types.

When vm.availableBlockTypes doesn't contain an element type that an area specifically references, the old code would just add an undefined to the allowedElementTypes. This causes the "add content" overlay to not open anymore.

[github-actions]

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[LennardF1989]

The CodeScene check failing is not really something I can be held responsible for, since it's already way over the allowed treshold to begin with :P

[mikecp]

Hi @LennardF1989 ,

Nice catch, and thank you for providing the fix for it 👍

Cheers!

[nul800sebastiaan]

Cherry picked for 10.5 in 9d11401