Fix error if no config file

[semuadmin]

PyGPSClient Pull Request Template

Description

Fix ValueError if pygpsclient started with no default configuration file. Also fixes issue if user cancels out of load config dialog.

Fixes #63

Testing

Please test all changes, however trivial, against the supplied pytest suite tests/test_*.py. Please describe any test cases you have amended or added to this suite to maintain >= 99% code coverage.

• [x] tested with and without default config file & if user cancels out of load config dialog

Checklist:

• [x] I agree to abide by the code of conduct (see CODE_OF_CONDUCT.md).
• [x] My code follows the style guidelines of this project (see CONTRIBUTING.MD).
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] (if appropriate) I have added test cases to the tests/test_*.py unittest suite to maintain >= 99% code coverage.
• [x] I have tested my code against the full tests/test_*.py unittest suite.
• [x] My changes generate no new warnings.
• [x] Any dependent changes have been merged and published in downstream modules.
• [x] I have signed my commits.
• [x] I understand and acknowledge that the code will be published under a BSD 3-Clause license.