Fix GetMTime calls when renderable is missing

Rendering/RayTracing/vtkOSPRayActorNode.cxx

@@ -153,6 +153,10 @@ vtkMTimeType vtkOSPRayActorNode::GetMTime()
 {
   vtkMTimeType mtime = this->Superclass::GetMTime();
   vtkActor* act = (vtkActor*)this->GetRenderable();
+  if (!act)
+  {
+    return mtime;
+  }
   if (act->GetMTime() > mtime)
   {
     mtime = act->GetMTime();

Rendering/RayTracing/vtkOSPRayVolumeNode.cxx

@@ -53,6 +53,11 @@ vtkMTimeType vtkOSPRayVolumeNode::GetMTime()
 {
   vtkMTimeType mtime = this->Superclass::GetMTime();
   vtkVolume* vol = (vtkVolume*)this->GetRenderable();
+  if (!vol)
+  {
+    return mtime;
+  }
+
   if (vol->GetMTime() > mtime)
   {
     mtime = vol->GetMTime();