Make sure that throwing an exception in an observer does not leave the PropertiesChanged mixin in an invalid state

[balint777]

…e PropertiesChanged mixin in an invalid state

Reference Issue

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[balint777]

@googlebot I signed it!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.