When lots of files are open (about 15) then I click 
the 'X' in the top right of the editor window to close 
them ALL. Occasionally 1 file won't close and will throw a 
IllegalArgumentException with the following stack trace:

----------------------------
java.lang.IllegalArgumentException: Cannot set student [Up-
to-date; 1.1.1.1] as selected. It's not present in the 
container.
	at 
org.netbeans.core.windows.frames.DefaultContainerImpl.setSe
lectedTopComponent(DefaultContainerImpl.java:923)
[catch] at 
org.netbeans.core.windows.frames.TabHandlePopupListener$1.r
un(TabHandlePopupListener.java:112)
	at java.awt.event.InvocationEvent.dispatch
(InvocationEvent.java:178)
	at java.awt.EventQueue.dispatchEvent
(EventQueue.java:448)
	at 
java.awt.EventDispatchThread.pumpOneEventForHierarchy
(EventDispatchThread.java:197)
	at 
java.awt.EventDispatchThread.pumpEventsForHierarchy
(EventDispatchThread.java:150)
	at java.awt.EventDispatchThread.pumpEvents
(EventDispatchThread.java:144)
	at java.awt.EventDispatchThread.pumpEvents
(EventDispatchThread.java:136)
	at java.awt.EventDispatchThread.run
(EventDispatchThread.java:99)
----------------------------

And the tab will not disappear but the window is empty 
(i.e. the file contents are not being shown).

The problem here is that you not cannot open the file that 
had BEEN in that tab, or else it will cause an index array 
out of bounds exception > 1 I think it is. This exception 
was thrown the first 3 times I tried to open the file, now 
it doesn't seem to be thrown naymore when I try and OPEN 
the file. However attempting to close the tab by right 
clicking will ALWAYS throw the exception.

Closing the X in the top right of the editors doesn't do 
anything either.

And pressing CTRL-F4 when the tab is focused doesn't do 
anything. I will attach my log file.