Fix poll error after 3de9915

The error was
Traceback (most recent call last):
  File "\blenderbim\bim\module\model\covering.py", line 64, in poll
    return element.is_a("IfcCovering")
AttributeError: 'NoneType' object has no attribute 'is_a'

src/blenderbim/blenderbim/bim/module/model/covering.py

@@ -61,7 +61,7 @@ class RegenSelectedFlooringObject(bpy.types.Operator, tool.Ifc.Operator):
     def poll(cls, context):
         active_obj = bpy.context.active_object
         element = tool.Ifc.get_entity(active_obj)
-        return element.is_a("IfcCovering")
+        return element and element.is_a("IfcCovering")
 
     def _execute(self, context):