Check source project is existing before editing merge request

app/models/merge_request.rb

@@ -946,6 +946,7 @@ class MergeRequest < ApplicationRecord
   end
 
   def can_remove_source_branch?(current_user)
+    source_project &&
       !ProtectedBranch.protected?(source_project, source_branch) &&
       !source_project.root_ref?(source_branch) &&
       Ability.allowed?(current_user, :push_code, source_project) &&

spec/models/merge_request_spec.rb

@@ -1170,6 +1170,12 @@ describe MergeRequest do
       expect(subject.can_remove_source_branch?(user)).to be_falsey
     end
 
+    it "can't be removed because source project has been deleted" do
+      subject.source_project = nil
+
+      expect(subject.can_remove_source_branch?(user)).to be_falsey
+    end
+
     it "can't remove a root ref" do
       subject.update(source_branch: 'master', target_branch: 'feature')