Werk #15491: Fix possible crash on editing pagetypes

If you edited pagetypes like topics, custom graphs, etc. and tried to save an unallowed value, e.g. float instead of integer for "Sort order", the error "UnboundLocalError: cannot access local variable 'new_page_dict' where it is not associated with a value" occurred.

You will now get a message which value is wrong to correct it before saving the pagetype.

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