added check for null events to prevent problems with null events when switching display methods. Not clear why this is necessary however.
added check for null events to prevent problems with null events when switching display methods. Not clear why this is necessary however.

git-svn-id: fe6b3b33f0410f5f719dcd9e0c58b92353e7a5d3@6650 b7f4320f-462c-0410-a916-d9f35bb82d52

SensorsINI/jaer,viktorbahr/jaer,viktorbahr/jaer,viktorbahr/jaer,SensorsINI/jaer,viktorbahr/jaer,SensorsINI/jaer,viktorbahr/jaer,viktorbahr/jaer,viktorbahr/jaer,SensorsINI/jaer,SensorsINI/jaer,SensorsINI/jaer,SensorsINI/jaer,SensorsINI/jaer