I was closing a large project and an error occured. The error dialog did not display itself, but this was printed to the Console:
Unhandled exception: java.lang.IllegalStateException: Interpreter JVM exited before registering, status: 1
Exception in thread "Wait for Interactions to Exit Thread" java.lang.NoClassDefFoundError: edu/rice/cs/drjava/ui/DrJavaErrorPopup$2
at edu.rice.cs.drjava.ui.DrJavaErrorPopup.<init>(DrJavaErrorPopup.java:114)
at edu.rice.cs.drjava.ui.DrJavaErrorHandler.record(DrJavaErrorHandler.java:93)
at edu.rice.cs.drjava.ui.DrJavaErrorHandler.handle(DrJavaErrorHandler.java:83)
at edu.rice.cs.drjava.model.repl.newjvm.MainJVM.slaveQuitDuringStartup(MainJVM.java:653)
at edu.rice.cs.util.newjvm.AbstractMasterJVM$1.run(AbstractMasterJVM.java:204)
Unhandled exception: java.lang.NoClassDefFoundError: edu/rice/cs/drjava/ui/DrJavaErrorWindow$3
Unhandled exception: java.lang.NoClassDefFoundError: edu/rice/cs/drjava/ui/DrJavaErrorWindow$3
Unhandled exception: java.lang.NoClassDefFoundError: edu/rice/cs/drjava/ui/RegionsTreePanel$4
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The "NoClassDefFoundErrors" look like there's something wrong with your build. Was this a custom build, or something that was released?
These kinds of errors can occur if you ctrl-c in the middle of building, then build again, because the dependency checking isn't perfect and sometimes doesn't realize that a certain step is only partially complete.
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dlsmith was right. A clean compile seems to have fixed it.