Description of problem:

che-server throws an error on workspace start on executing the oauth callback request.

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

 # <steps>

1. Start a workspace from a GitHub public repository, accept the authorisation as you can not reject it because of [this](https://github.com/eclipse-che/che-server/pull/568) bug.
2. Stop the workspace.
3. Go to dashboard -> user preferences -> git services and revoke the authorisation.
4. Start the workspace and reject the authorisation.

Actual results:

an error appears

Expected results:

workspace starts successfully.

Reproducibility (Always/Intermittent/Only Once):

Acceptance criteria: 

Definition of Done:

Build Details:

Additional info (Such as Logs, Screenshots, etc):

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