Drop 2 Buttons and select them (multi-selection).
Set the fx:id to "test" from Properties.
We get 2 messages "An Element with the id test is already injected...", where a single one is enough.
Select now the 2nd button to change its fx:id, and set it to "test2" : the value is erased and become empty.
The fx:id of the 1st Button is also empty.
Save it : the fx:id is "test" for the 2 Buttons...
We have several issue there, one of them being an inspector refresh issue.

I also had the tool hang playing around this, but was not able to reproduce.