Fix segfault on missing Anchor view

- test script did not create Anchor(Front) view.  DPG/DVP did not
  handle missing Anchor properly

src/Mod/TechDraw/App/DrawViewPart.cpp

@@ -239,10 +239,18 @@ App::DocumentObjectExecReturn *DrawViewPart::execute(void)
     if (!keepUpdated()) {
         return App::DocumentObject::StdReturn;
     }
+    
+    const std::vector<App::DocumentObject*>& links = Source.getValues();
+    if (links.empty())  {
+        Base::Console().Log("DVP::execute - %s - No Sources - creation time?\n",getNameInDocument());
+        return App::DocumentObject::StdReturn;
+    }
 
-    TopoDS_Shape shape = getSourceShape();
+    TopoDS_Shape shape = getSourceShape();          //if shape is null, it is probably obj creation time.
     if (shape.IsNull()) {
-        return new App::DocumentObjectExecReturn("DVP - Linked shape object is invalid");
+        Base::Console().Log("DVP::execute - %s - source shape is invalid - creation time?\n",
+                            getNameInDocument());
+        return App::DocumentObject::StdReturn;
     }
 
     gp_Pnt inputCenter;