AttributeError: 'NoneType' object has no attribute 'get_similar_commands'

[Ryan J Ollos]

When running the command:

trac-admin $env help $cmd

the following output results for the case that $env is not a valid environment and $cmd is not a valid command:

$ trac-admin noenv help nocmd
No documentation found for 'nocmd'. Use 'help' to see the list of commands.
AttributeError: 'NoneType' object has no attribute 'get_similar_commands'

Prior to [12277], a different error would result:

$ trac-admin noenv help nocmd
No documentation found for 'nocmd'. Use 'help' to see the list of commands.
UnboundLocalError: local variable 'cmd_mgr' referenced before assignment

Thanks to Olemis Lang for initial investigating of the issue, which was reported in bloodhound:#762.