fix click_source_redirect method on doc_methods_controller

[GabrielRMuller]

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While making tests for the class (#943), I've noticed that this method wasn't working properly because of two errors:

• on line 36 the search for a RepoSubscription was not being made correctly, which would consequently lead to the sub variable being nil.
• on line 39 the if conditional was raising an error in case the condition wasn't met instead of just executing the else instruction. To fix it I've prevented the condition of raising an error.