fix(Pointer): prevent crash when closing steam menu

There was an issue where the boundary renderers would not be
available when the Steam in game menu was activated because
the mesh would be recreated when the game menu was closed.

Assets/VRTK/Scripts/Pointers/VRTK_PlayAreaCursor.cs

@@ -235,7 +235,10 @@ public virtual void ToggleVisibility(bool state)
 
             for (int i = 0; i < boundaryRenderers.Length; i++)
             {
-                boundaryRenderers[i].enabled = state;
+                if (boundaryRenderers[i] != null)
+                {
+                    boundaryRenderers[i].enabled = state;
+                }
             }
         }