A validation error can occur when submitting the OAuth app form (such as when 

redirect URIs are invalid URLs). When these validation errors occur, it messes

up the flow between ListEditField and ListEditWidget (which are the field and

widget for the redirect URIs field) and causing a value error.
On a normal basis, when submitting the form, the 

ListEditWidget.value_from_datadict method is called, which extracts the

redirect URIs data from the form and returns a list of redirect URIs. Then this

list is passed to ListEditField.to_python, which takes the list and transforms

it into a string of comma separated redirect URIs, and this gets stored in the

database. When the page reloads after the form is submitted, 

ListEditField.prepare_value is called to take the string from the database and

turn it into a list to pass to the widget for when it renders. This is good.
When a validation error occurs, the list is never passed to 

ListEditField.to_python because we're not storing in the database since

there's an error. So instead when the page reloads, the list from 

ListEditWidget.value_from_datadict is passed to 

ListEditField.prepare_value, which only accepts strings or None, and this

is why we're getting the value error.
To fix this, we allow ListEditField.prepare_value to handle lists. We make

this change to ListEditDictionaryField.prepare_value as well, so that it

can also handle cases where a form that contains the field errors out.

Ran tests in djblets/forms/tests
Ran tests in /reviewboard/webapi/tests/testauth_app.py
Ran tests in ./reviewboard/admin/tests/test_lexers_mapping_widget.py
Manually tested the OAuth Application form with combinations of valid and

  invalid redirect URIs and blank URIs