Fix Error 500 when inviting user to a few projects

changelogs/unreleased/214691-error-500-when-inviting-user-to-a-few-projects.yml

+---
+title: Fix Error 500 when inviting user to a few projects
+merge_request: 29778
+author:
+type: fixed

ee/lib/gitlab/auth/group_saml/gma_membership_enforcer.rb

@@ -22,6 +22,7 @@ module Gitlab
 
         def check_source_project_membership(user)
           return true unless project.forked?
+          return true unless project.forked_from_project
 
           check_group_managed_account(project.forked_from_project.root_ancestor, user)
         end

ee/spec/lib/gitlab/auth/group_saml/gma_membership_enforcer_spec.rb

@@ -50,6 +50,29 @@ describe Gitlab::Auth::GroupSaml::GmaMembershipEnforcer do
     end
   end
 
+  context 'when the project is forked from deleted project' do
+    let!(:forked_project) { fork_project(project, managed_user_for_project) }
+
+    before do
+      project.add_developer(managed_user_for_project)
+      project.delete
+    end
+
+    context 'when user is group-managed' do
+      it 'allows adding user to project' do
+        subject = described_class.new(forked_project)
+        expect(subject.can_add_user?(managed_user)).to be_truthy
+      end
+    end
+
+    context 'when user is not group-managed' do
+      it 'does not allow adding user to project' do
+        subject = described_class.new(forked_project)
+        expect(subject.can_add_user?(create(:user))).to be_truthy
+      end
+    end
+  end
+
   context 'when project is forked from namespace to group' do
     let(:project) { create(:project) }
     let(:forked_project) { create(:project, namespace: group) }