BUG: Fix small bugs in External Beam Planning

- When loading a scene a new markups POI node was created, because a temporary nullptr setting in the POI markups node combobox
- When plan node is selected that is not empty the instructions text is now cleared

Re #73

ExternalBeamPlanning/qSlicerExternalBeamPlanningModuleWidget.cxx

@@ -424,6 +424,9 @@ void qSlicerExternalBeamPlanningModuleWidget::setPlanNode(vtkMRMLNode* node)
         firstBeamNode->InvokeCustomModifiedEvent(vtkMRMLRTBeamNode::BeamTransformModified);
       }
     }
+
+    // Clear instructions text
+    d->label_CalculateDoseStatus->setText("");
   }
 
   this->updateWidgetFromMRML();
@@ -514,6 +517,12 @@ void qSlicerExternalBeamPlanningModuleWidget::poisMarkupsNodeChanged(vtkMRMLNode
   planNode->SetAndObservePoisMarkupsFiducialNode(vtkMRMLMarkupsFiducialNode::SafeDownCast(node));
   planNode->DisableModifiedEventOff();
 
+  if (!node)
+  {
+    // Do not update beams or isocenter if no markups node was given
+    return;
+  }
+
   // Update beam transforms based on new isocenter
   std::vector<vtkMRMLRTBeamNode*> beams;
   planNode->GetBeams(beams);