FIX #4: "PHP Notice: Undefined variable: result" when installing plugin

Made sure $result is correctly set as it should be.
FIX #5: "PHP Notice: Undefined index: field" when doing a mass action

Added missing "field" index for "Alternate username" criteria
Merge branch 'bugfix/5-undefined_index_field_when_massaction' into develop
Merge branch 'bugfix/4-undefined_result_when_installing' into develop