Fix welcome message crashing server if podmin is set to invalid user.

Fixes #7363

closes #7452

Changelog.md

@@ -8,6 +8,7 @@
 * Fix first comment in mobile view with french locale [#7441](https://github.com/diaspora/diaspora/pull/7441)
 * Use post page title and post author in atom feed [#7420](https://github.com/diaspora/diaspora/pull/7420)
 * Handle broken public keys when receiving posts [#7448](https://github.com/diaspora/diaspora/pull/7448)
+* Fix welcome message when podmin is set to an invalid username [#7452](https://github.com/diaspora/diaspora/pull/7452)
 
 ## Features
 

app/models/user.rb

@@ -431,6 +431,7 @@ class User < ActiveRecord::Base
     return unless AppConfig.settings.welcome_message.enabled? && AppConfig.admins.account?
     sender_username = AppConfig.admins.account.get
     sender = User.find_by(username: sender_username)
+    return if sender.nil?
     conversation = sender.build_conversation(
       participant_ids: [sender.person.id, person.id],
       subject:         AppConfig.settings.welcome_message.subject.get,

spec/models/user_spec.rb

@@ -875,6 +875,12 @@ describe User, :type => :model do
         user.send_welcome_message
         expect(user.conversations.count).to eq 0
       end
+
+      it "should send no welcome message if podmin is invalid" do
+        AppConfig.admins.account = "invalid"
+        user.send_welcome_message
+        expect(user.conversations.count).to eq 0
+      end
     end
 
     context "with welcome message disabled" do