Fix notify_conflict? raising exception when branches do not exist

repository.can_be_merged? can raise error if diff_head_sha or
target_branch are absent, therefore check those explicitly.

app/models/merge_request.rb

@@ -128,14 +128,9 @@ class MergeRequest < ActiveRecord::Base
     end
 
     after_transition unchecked: :cannot_be_merged do |merge_request, transition|
-      begin
-        if merge_request.notify_conflict?
-          NotificationService.new.merge_request_unmergeable(merge_request)
-          TodoService.new.merge_request_became_unmergeable(merge_request)
-        end
-      rescue Gitlab::Git::CommandError
-        # Checking mergeability can trigger exception, e.g. non-utf8
-        # We ignore this type of errors.
+      if merge_request.notify_conflict?
+        NotificationService.new.merge_request_unmergeable(merge_request)
+        TodoService.new.merge_request_became_unmergeable(merge_request)
       end
     end
 
@@ -707,7 +702,14 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def notify_conflict?
-    (opened? || locked?) && !project.repository.can_be_merged?(diff_head_sha, target_branch)
+    (opened? || locked?) &&
+      has_commits? &&
+      !branch_missing? &&
+      !project.repository.can_be_merged?(diff_head_sha, target_branch)
+  rescue Gitlab::Git::CommandError
+    # Checking mergeability can trigger exception, e.g. non-utf8
+    # We ignore this type of errors.
+    false
   end
 
   def related_notes

changelogs/unreleased/48653-mr-target-branch-missing.yml

+---
+title: Fix merge request page rendering error when its target/source branch is missing
+merge_request: 20280
+author:
+type: fixed

spec/models/merge_request_spec.rb

@@ -2190,6 +2190,22 @@ describe MergeRequest do
           end
         end
       end
+
+      context 'source branch is missing' do
+        subject { create(:merge_request, :invalid, :opened, merge_status: :unchecked, target_branch: 'master') }
+
+        before do
+          allow(subject.project.repository).to receive(:can_be_merged?).and_call_original
+        end
+
+        it 'does not raise error' do
+          expect(notification_service).not_to receive(:merge_request_unmergeable)
+          expect(todo_service).not_to receive(:merge_request_became_unmergeable)
+
+          expect { subject.mark_as_unmergeable }.not_to raise_error
+          expect(subject.cannot_be_merged?).to eq(true)
+        end
+      end
     end
 
     describe 'check_state?' do