Fixed crash in regressiontest of statistics case

Destructor chain deleting view fired off getting all cases, which
accessed case during its destruction.

ApplicationCode/UserInterface/RiuMainWindow.cpp

@@ -1327,6 +1327,7 @@ void RiuMainWindow::slotSubWindowActivated(QMdiSubWindow* subWindow)
 {
     RimProject * proj = RiaApplication::instance()->project();
     if (!proj) return;
+    if (!subWindow) return;
 
     // Iterate all cases in each oil field
     std::vector<RimCase*> allCases;