Trying 15.0.0 32 bit.
If the test window session was killed and I accidentally push "start debugger" button then I get the correct message "not connected to Oracle".
But after that it looks like the debug session is not terminated - start button is disabled and run button and others are enabled, the window is in executing state.
I can't neither break execution nor close window nor compile any program unit nor relog nor exit the application correctly, followed by the same message.

We'll check it out and will try to fix it.

Was there an update on this? I've had this happen twice recently, and the only way out was to kill the app from task manager.