Fix crash on exit as "Fact" was calling a nullified qgcApp().

src/FactSystem/Fact.cc

@@ -733,6 +733,7 @@ FactValueSliderListModel* Fact::valueSliderModel(void)
 
 void Fact::_checkForRebootMessaging(void)
 {
+    if(qgcApp()) {
         if (!qgcApp()->runningUnitTests()) {
             if (vehicleRebootRequired()) {
                 qgcApp()->showMessage(tr("Change of parameter %1 requires a Vehicle reboot to take effect.").arg(name()));
@@ -741,3 +742,4 @@ void Fact::_checkForRebootMessaging(void)
             }
         }
     }
+}