[apps] Less operations in willExitResponderChain if exiting app

If nextFirstResponder is nullptr, it means we are exiting the app, so
there is no need to relayout, reselect, ...
We did it bbefore because a long time ago, we used to keep all apps side
by side so they had to be thouroughly cleaned when exiting

apps/statistics/histogram_controller.cpp

@@ -70,7 +70,8 @@ void HistogramController::viewWillAppear() {
 }
 
 void HistogramController::willExitResponderChain(Responder * nextFirstResponder) {
-  if (nextFirstResponder == nullptr || nextFirstResponder == tabController()) {
+  if (nextFirstResponder == tabController()) {
+    assert(tabController() != nullptr);
     if (selectedSeriesIndex() >= 0) {
       m_view.dataViewAtIndex(selectedSeriesIndex())->setForceOkDisplay(false);
     }