[KevinH]

In case this helps, I changed your use of "try:" to catch a ValueError to instead catch all Exceptions and print out the exceptions error message and I get the following errors when trying to change the Preview Timeout and then save:

Traceback (most recent call last):
  File "/Users/kbhend/Library/Application Support/sigil-ebook/sigil/plugins/EnvVarEditor/plugin.py", line 849, in on_accept
    values = self.get_values()
  File "/Users/kbhend/Library/Application Support/sigil-ebook/sigil/plugins/EnvVarEditor/plugin.py", line 790, in get_values
    if w.value() != default_val:
                    ^^^^^^^^^^^
NameError: name 'default_val' is not defined

Hope this helps.