blenderbim - hide json output file and some operators if current ifc file is not used

It was misleading since those operators only worked with the current model and had no effect if it wasn't used in ifcdiff.

src/blenderbim/blenderbim/bim/module/diff/ui.py

@@ -98,6 +98,9 @@ class BIM_PT_diff(Panel):
         row = layout.row()
         row.operator("bim.execute_ifc_diff")
 
+        if props.active_file == "NONE":
+            return
+
         row = layout.row(align=True)
         row.prop(props, "diff_json_file")
         row.operator("bim.select_diff_json_file", icon="FILE_FOLDER", text="")