This bug was originally marked as duplicate of bug 190136, that is already resolved. This bug is still valid, so this seems to be another bug, but it might be related.
Build: NetBeans IDE Dev (Build 201609190002)
VM: Java HotSpot(TM) 64-Bit Server VM, 25.45-b02, Java(TM) SE Runtime Environment, 1.8.0_45-b14
tbrunhoff: stepping in the debugger.
Created attachment 162278 [details]
This is a result of a race condition. See #190136 - the UI code needs to be called in AWT thread.
fixed on enum
user: Maria Dalmatova <email@example.com>
date: Thu Dec 08 11:57:20 2016 +0300
summary: fixed bz#268278 - NullPointerException at org.netbeans.editor.ext.ToolTipSupport.ensureVisibility
Integrated into 'main-silver', will be available in build *201612100001* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
User: Maria Dalmatova <firstname.lastname@example.org>
Log: fixed bz#268278 - NullPointerException at org.netbeans.editor.ext.ToolTipSupport.ensureVisibility
(transplanted from e21148930fc1f02bb3b23d1b3118ed9ebaead855)
I can't reproduce this random exception in internal NetBeans 8,2 patch 1 build.