More NULL ride bug fixes

The controls for the RideEditor (find) and AllPlotWindow
still try and do their thing when the current ride item
is NULL. This was either because it wasn't checked (or
expected) or the widgets in question were not notified. This
patch fixes this.

When I get a chance the whole codebase needs to be refactored
to gracefully handle NULL rideItems -- each plot/window should
clear state when it is notified that NO ride is selected.

src/MainWindow.cpp

@@ -897,6 +897,12 @@ MainWindow::removeCurrentRide()
     rideDeleted(item);
     delete item;
 
+    // any left?
+    if (allRides->childCount() == 0) {
+        ride = NULL;
+        rideTreeWidgetSelectionChanged(); // notifies children
+    }
+
     // added djconnel: remove old cpi file, then update bests which are associated with the file
     criticalPowerWindow->deleteCpiFile(strOldFileName);