BIM: Avoid traceback if there is no Classification selected

As the title says, if user selects `OK` button and has no Classification
selected (which can happen only if the list is empty I think), then they
get a traceback since we're trying to access list of selections
directly.

So this patch adds a small patch to close the form if user has pressed
`OK` button and did not select anything.

src/Mod/BIM/bimcommands/BimClassification.py

@@ -624,6 +624,12 @@ class BIM_Classification:
                 FreeCAD.ActiveDocument.commitTransaction()
                 FreeCAD.ActiveDocument.recompute()
         else:
+            # Close the form if user has pressed Enter and did not
+            # select anything
+            if len(self.form.treeClass.selectedItems()) < 1:
+                self.form.close()
+                return
+
             code = self.form.treeClass.selectedItems()[0].text(0)
             pl = self.isEditing.PropertiesList
             if ("StandardCode" in pl) or ("IfcClass" in pl):