Fix crashing when hitting tab in texture edit mode

src/Application/TextureMappingState.cpp

@@ -35,10 +35,12 @@ void TextureMappingState::onKeyPressed(Application * app, ofKeyEventArgs & args)
 		 break;
 		 
 	 case OF_KEY_TAB:
+		 if(app->getSurfaceManager()->getSelectedSurface() != 0){
 			app->getCmdManager()->exec(
 				new SetNextSourceCmd(
 					app->getSurfaceManager()->getSelectedSurface(),
 					app->getGui()->getSourcesEditor()));
+		 }
 		 break;
 	}