Exception when Plot Reference is checked and swing fails

[csatt]

If the Plot Reference checkbox is checked and swinging a curve fails due to an error such as Voc = 0, the dialog with the error cause is not displayed and instead, an exception dialog is displayed. The exception looks like this in the log file:

210629_16_03_56: ERROR: Voc is zero volts
210629_16_03_56: Unexpected error: <class '_tkinter.TclError'>
210629_16_03_56: Traceback (most recent call last):
File "tkinter/init.py", line 1883, in call
File "IV_Swinger2_gui.py", line 1843, in go_button_actions
File "IV_Swinger2_gui.py", line 1880, in go_actions
File "IV_Swinger2_gui.py", line 1990, in swing_loop
File "IV_Swinger2_gui.py", line 2040, in display_img
File "tkinter/init.py", line 4061, in init
File "tkinter/init.py", line 4006, in init
_tkinter.TclError: value for "-file" missing