Fix crashing on mouse release if no source selected

- SurfaceManagerGui mouseRelease method did not check whether a surface is selected before checking if a selected surface has been moved

src/Surfaces/SurfaceManagerGui.cpp

@@ -199,9 +199,11 @@ namespace ofx {
             textureEditor.stopDragJoints();
             
             // Check if surface has moved
+            if (surfaceManager->getSelectedSurface()){
                 if (!surfaceManager->getSelectedSurface()->getMoved()) {
                     _cmdManager->undo();
                 }
+            }
             
         }