Avoid error 500 when todo author was removed

app/helpers/application_helper.rb

@@ -72,7 +72,7 @@ module ApplicationHelper
     if user_or_email.is_a?(User)
       user = user_or_email
     else
-      user = User.find_by(email: user_or_email.downcase)
+      user = User.find_by(email: user_or_email.try(:downcase))
     end
 
     if user

app/views/dashboard/todos/_todo.html.haml

@@ -4,7 +4,10 @@
 
     .todo-title
       %span.author-name
-        = link_to_author todo
+        - if todo.author
+          = link_to_author(todo)
+        - else
+          (removed)
       %span.todo-label
         = todo_action_name(todo)
         = todo_target_link(todo)