Created attachment 136269 [details]
log from .xsession-errors

trying to create a reproduceable project, but the following procedure
- killed the X window, netbeans is still running.
- crashed the JVM
- freeze NB

how it comes:

create New Project... -> Java Class Library -> name it TestBase -> ok
create new Java Package under source -> name it gui -> ok
create new Java Class... -> name it MyLabel -> ok
in editor edit the class so it extends from JLabel and calls super("MyLabel");
Save all

create New Project... -> Java Application -> name it TestApp -> ok
create new JPanel Form... -> name it MyPanel -> ok -> panel in editor
TestApp>Properties>Libraries>Add Project... -> select TestBase -> ok
drag TestBase>Source>gui>MyLabel from Project into Design Editor -> Warning! ->
ok
compile File MyLabel (wrong, must compile MyPanel why?)
drag TestBase>Source>gui>MyLabel from Project into Design Editor -> Warning!
---- crash XWindow gone, Jave still alive
this is reproduceable
in TestApp create a new Panel Form
drag MyLabel into -> warning -> ok
drag MyLabel into -> warning -> ok
---- crash Netbeans gone