[Task] Solve the issue with Bragg panel after resetting

1. Pin down the cause for the issue.

   Traceback (most recent call last):
     File "/SNS/users/y8z/miniconda/envs/addie_env_3/lib/python3.6/site-packages/addie/main.py", line 692, in evt_switch_bragg_unit
       rietveld_event_handler.switch_bragg_unit(main_window=self)
     File "/SNS/users/y8z/miniconda/envs/addie_env_3/lib/python3.6/site-packages/addie/rietveld/event_handler.py", line 377, in switch_bragg_unit
       assert len(ws_group_list) > 0, 'At least 1 GSS file must be selected.'
   AssertionError: At least 1 GSS file must be selected.
   Aborted (core dumped)

   • It seems the program is expecting the number of workspaces (those shown in the Bragg Workspaces panel) to be non-zero.

   • However, after resetting the GSAS-tab, the number of workspaces already become zero and therefore in this case we need to disable all the plot control buttons/checkboxes (just like when we first launch the GUI, all those buttons/checkboxes are greyed out before loading in any data).

2. Solve the issue.

   • Locate where the unit selection dropdown menu is defined.

   Searching for dSpacing in the codes -> from line-29 in addie/initialization/widgets/rietveld_tab.py, we know it is one of the options in comboBox_xUnit.

   • Locate comboBox_xUnit in the codes.

   We know that definitely this comboBox_xUnit should have been disabled during the initialization stage, so we want to look into the codes to see how this was done.

   At line-199 in addie/rietveld/event_handler.py, we see ui.comboBox_xUnit.setEnabled(enable_widgets) which is definitely what we want.

   • We need to call check_rietveld_widgets method after resetting the GSAS-tab.

   Go to addie/rietveld/braggtree.py and go to the end of do_reset_gsas_tab method. We need to put the following lines to the end of do_reset_gsas_tab method,

   main_window._onCanvasGSSBankList = []
   event_handler.check_rietveld_widgets(main_window)

   The first line added above is to reset the _onCanvasGSSBankList variable so that check_rietveld_widgets method knows that we have an empty workspace list.