Fixed|Widgets|libappfw: Potential crash when opening a panel widget

In the rare situation where several assets changed their state during
the same event triggering loop, PanelWidget may have accessed a null
pointer.

doomsday/sdk/libappfw/src/widgets/panelwidget.cpp

@@ -190,7 +190,7 @@ DENG_GUI_PIMPL(PanelWidget)
         LOG_AS("PanelWidget");
 
         // All of the assets in the pending show group are now ready, let's open!
-        if (pendingShow->isReady())
+        if (pendingShow && pendingShow->isReady())
         {
             LOGDEV_XVERBOSE("All assets ready, resuming animation", "");