"Then I can edit the following fields" Step Binding throws 'Object reference not set to an instance of an object.'

## Describe the bug
When using the "Then I can edit the following fields" step binding, if a given field cannot be found, an 'Object reference not set to an instance of an object.' exception is thrown instead of a concise error detailing which field is not editable.

## Expected behaviour
The exception is caught, and an error is thrown detailing which field could not be found.

## Environment (please complete the following information):
- Test framework - MSTest]
- Browser - chrome
- Version - 92
- Power Apps environment version - 9.2.21073.00130