[FEM] improve checks for datasets

- check if datasets exists before we access them
- speed up the check for the datatype by testing after the downcast

src/Mod/Fem/App/FemPostObject.cpp

@@ -48,8 +48,9 @@ vtkBoundingBox FemPostObject::getBoundingBox() {
 
     vtkBoundingBox box;
 
-    if (Data.getValue() && Data.getValue()->IsA("vtkDataSet"))
-        box.AddBounds(vtkDataSet::SafeDownCast(Data.getValue())->GetBounds());
+    vtkDataSet* dset = vtkDataSet::SafeDownCast(Data.getValue());
+    if (dset)
+        box.AddBounds(dset->GetBounds());
 
     // TODO: add calculation of multiblock and Multipiece datasets