Fix error when MR source branch no longer exists.

app/controllers/projects/merge_requests_controller.rb

@@ -258,7 +258,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @commits = @merge_request.commits
 
     @merge_request_diff = @merge_request.merge_request_diff
-    @source_branch = @merge_request.source_project.repository.find_branch(@merge_request.source_branch).try(:name)
     
     if @merge_request.locked_long_ago?
       @merge_request.unlock_mr

app/views/projects/merge_requests/widget/_merged.html.haml

@@ -20,7 +20,7 @@
               The changes were merged into
               %span.label-branch= @merge_request.target_branch
             You can remove the source branch now.
-          = link_to namespace_project_branch_path(@merge_request.source_project.namespace, @merge_request.source_project, @source_branch), remote: true, method: :delete, class: "btn btn-primary btn-sm remove_source_branch" do
+          = link_to namespace_project_branch_path(@merge_request.source_project.namespace, @merge_request.source_project, @merge_request.source_branch), remote: true, method: :delete, class: "btn btn-primary btn-sm remove_source_branch" do
             %i.fa.fa-times
             Remove Source Branch