fix(UI): prevent crash when no pointer is attached to UI Pointer

There was an issue where the VRInputModule would crash if no controller
was attached to the UI Pointer because it was trying to access a
controller to get the touchpad axis data for the scroll amount.

This has been fixed by checking for the existence of a controller.

Assets/VRTK/Source/Scripts/Internal/VRTK_VRInputModule.cs

@@ -316,7 +316,7 @@ protected virtual void Drag(VRTK_UIPointer pointer, List<RaycastResult> results)
 
         protected virtual void Scroll(VRTK_UIPointer pointer, List<RaycastResult> results)
         {
-            pointer.pointerEventData.scrollDelta = pointer.controller.GetTouchpadAxis();
+            pointer.pointerEventData.scrollDelta = (pointer.controller != null ? pointer.controller.GetTouchpadAxis() : Vector2.zero);
             bool scrollWheelVisible = false;
             for (int i = 0; i < results.Count; i++)
             {