Fixes #3741 - Removal of Microsoft365-Channel not possible via UI when having no email address relation.

app/controllers/channels_microsoft365_controller.rb

@@ -55,7 +55,7 @@ class ChannelsMicrosoft365Controller < ApplicationController
   def destroy
     channel = Channel.find_by(id: params[:id], area: 'Microsoft365::Account')
     email   = EmailAddress.find_by(channel_id: channel.id)
-    email.destroy!
+    email&.destroy!
     channel.destroy!
     render json: {}
   end

spec/requests/channels_microsoft365_spec.rb

@@ -0,0 +1,50 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+
+RSpec.describe 'Microsoft365 channel API endpoints', type: :request do
+  let(:admin) { create(:admin) }
+  let!(:microsoft365_channel) { create(:microsoft365_channel) }
+
+  describe 'DELETE /api/v1/channels_microsoft365', authenticated_as: :admin do
+    context 'without a email address relation' do
+      let(:params) do
+        {
+          id: microsoft365_channel.id
+        }
+      end
+
+      it 'responds 200 OK' do
+        delete '/api/v1/channels_microsoft365', params: params, as: :json
+
+        expect(response).to have_http_status(:ok)
+      end
+
+      it 'microsoft365 channel deleted' do
+        expect { delete '/api/v1/channels_microsoft365', params: params, as: :json }.to change(Channel, :count).by(-1)
+      end
+    end
+
+    context 'with a email address relation' do
+      let(:params) do
+        {
+          id: microsoft365_channel.id
+        }
+      end
+
+      before do
+        create(:email_address, channel: microsoft365_channel)
+      end
+
+      it 'responds 200 OK' do
+        delete '/api/v1/channels_microsoft365', params: params, as: :json
+
+        expect(response).to have_http_status(:ok)
+      end
+
+      it 'microsoft365 channel and related email address deleted' do
+        expect { delete '/api/v1/channels_microsoft365', params: params, as: :json }.to change(Channel, :count).by(-1).and change(EmailAddress, :count).by(-1)
+      end
+    end
+  end
+end