Correct small bug in CompoundFilter in command line

Without this, CompoundFilter does not work in Python script in non-GUI-mode.
See:
https://forum.freecadweb.org/viewtopic.php?f=10&t=27593&p=222303
and
https://www.freecadweb.org/tracker/view.php?id=3381

src/Mod/Part/CompoundTools/CompoundFilter.py

@@ -44,7 +44,8 @@ def makeCompoundFilter(name):
     '''makeCompoundFilter(name): makes a CompoundFilter object.'''
     obj = FreeCAD.ActiveDocument.addObject("Part::FeaturePython", name)
     _CompoundFilter(obj)
-    _ViewProviderCompoundFilter(obj.ViewObject)
+    if obj.ViewObject:    
+        _ViewProviderCompoundFilter(obj.ViewObject)
     return obj