Core: Reset current OutputPanePlaceHolder on deletion...

...to avoid nullptr access

Task-number: QTCREATORBUG-15987
Change-Id: I98da9323d13027192b77d0ba7ac8b846849930f0
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/outputpane.cpp

@@ -78,6 +78,7 @@ OutputPanePlaceHolder::~OutputPanePlaceHolder()
             om->setParent(0);
             om->hide();
         }
+        OutputPanePlaceHolderPrivate::m_current = nullptr;
     }
     delete d;
 }