Check if pipeline is nil before merging

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
+v 8.9.1 (unreleased)
+  - Fix in auto merge when pipeline is nil
+
 v 8.9.0 (unreleased)
   - Fix builds API response not including commit data
   - Fix error when CI job variables key specified but not defined

app/controllers/projects/merge_requests_controller.rb

@@ -205,7 +205,12 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @merge_request.update(merge_error: nil)
     if params[:merge_when_build_succeeds].present?
-      if @merge_request.pipeline && @merge_request.pipeline.active?
+      unless @merge_request.pipeline
+        @status = :failed
+        return
+      end
+
+      if @merge_request.pipeline.active?
         MergeRequests::MergeWhenBuildSucceedsService.new(@project, current_user, merge_params)
                                                         .execute(@merge_request)
         @status = :merge_when_build_succeeds