check valid pointer (#2674)

Signed-off-by: Ian Chen <ichen@openrobotics.org>

Author: iche033

src/systems/optical_tactile_plugin/OpticalTactilePlugin.cc

@@ -632,7 +632,7 @@ void OpticalTactilePluginPrivate::Enable(const gz::msgs::Boolean &_req)
 
   this->enabled = _req.data();
 
-  if (!_req.data())
+  if (!_req.data() && this->visualizePtr)
   {
     this->visualizePtr->RemoveNormalForcesAndContactsMarkers();
   }