Complete Code Coverage General View Models

[palisadoes]

The Talawa code base needs to be 100% reliable. This means we need to have 100% unittest code coverage. This is a parent issue for all Pre-Auth View Model unittest issues.

We will be creating one issue per related file with the expectation that widgets referenced in these files must also have unittests.

• We'll only assign the child issues.
• Please comment on one of the child issues for assignment.
• Only one child issue will be assigned to a person at any one time.

Note: 5 additional points will be assigned to any potential internship applicants who completes all the child issues.

Parent Issue:

• Complete Code Coverage for View Models #1001

Child Issues:

• Unittests for theme_view_model.dart #1120
• Unittests for main_screen_view_model.dart #1121
• Unittests for lang_view_model.dart #1122
• Unittests for base_view_model.dart #1123