How to delete all messages created using Mailboxer gem and start fresh?

[suramai]

I followed a tutorial and added messaging functionality in my rails app.
Yesterday, I deleted certain users. Now when I go to my messages link, I get this error message "We're sorry, but something went wrong. If you are the application owner check the logs for more information." which is usually displayed in case of errors.

I saw the log and found out that I must have deleted someone who has sent a message.
I think it's trying to retrieve avatar of a user that doesn't exist.

ActionView::Template::Error (undefined method `avatar' for nil:NilClass):

How can I solve this issue? How can I delete everything related to the users (with regards to mailboxer) that I have deleted from my website?
Please share how to delete everything associated to a user automatically when the user is deleted because everytime I delete a user I am forced to delete all the mailboxer conversations to avoid errors.

And could anyone please share how to delete all the messages and start fresh? Because the messages are not important.
How do I clear all the mailboxer tables and start fresh in heroku? Kindly help.

Here is the log.

2015-05-16T12:17:13.398640+00:00 app[web.1]:   Rendered conversations/_participants.html.erb (17.3ms)
2015-05-16T12:17:13.407905+00:00 app[web.1]:   CACHE (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 2225]]
2015-05-16T12:17:13.434486+00:00 app[web.1]:    (16.2ms)  SELECT COUNT(*) FROM "mailboxer_receipts" INNER JOIN "mailboxer_notifications" ON "mailboxer_notifications"."id" = "mailboxer_receipts"."notification_id" AND "mailboxer_notifications"."type" IN ('Mailboxer::Message') WHERE "mailboxer_notifications"."conversation_id" = 973 AND "mailboxer_receipts"."receiver_id" = $1 AND "mailboxer_receipts"."receiver_type" = $2 AND "mailboxer_receipts"."trashed" = 'f' AND "mailboxer_receipts"."is_read" = 'f'  [["receiver_id", 1464], ["receiver_type", "User"]]
2015-05-16T12:17:13.453617+00:00 app[web.1]:   Mailboxer::Receipt Load (1.1ms)  SELECT "mailboxer_receipts".* FROM "mailboxer_receipts" WHERE "mailboxer_receipts"."notification_id" = $1  [["notification_id", 1209]]
2015-05-16T12:17:13.456255+00:00 app[web.1]:   Rendered conversations/_participants.html.erb (20.5ms)
2015-05-16T12:17:13.474657+00:00 heroku[router]: at=info method=GET path="/conversations" host=www.abc.com request_id=991e2d24-c507-4e5f-9b50-5678e2e3aa2c fwd="160.3.94.130" dyno=web.1 connect=1ms service=276ms status=500 bytes=1786
2015-05-16T12:17:13.569003+00:00 heroku[router]: at=info method=GET path="/speak" host=www.abc.com request_id=c1a056f3-4431-4c90-84b8-01c285e606c1 fwd="122.174.14.148" dyno=web.2 connect=1ms service=37ms status=200 bytes=35624
2015-05-16T12:17:13.459247+00:00 app[web.1]:   Mailboxer::Message Load (1.8ms)  SELECT  "mailboxer_notifications".* FROM "mailboxer_notifications" WHERE "mailboxer_notifications"."type" IN ('Mailboxer::Message') AND "mailboxer_notifications"."conversation_id" = $1  ORDER BY created_at DESC LIMIT 1  [["conversation_id", 782]]
2015-05-16T12:17:13.469138+00:00 app[web.1]:   Rendered conversations/_conversation.html.erb (233.8ms)
2015-05-16T12:17:13.469485+00:00 app[web.1]:   Rendered conversations/index.html.erb within layouts/application (249.4ms)
2015-05-16T12:17:13.469543+00:00 app[web.1]: Completed 500 Internal Server Error in 261ms
2015-05-16T12:17:13.473924+00:00 app[web.1]: 
2015-05-16T12:17:13.473928+00:00 app[web.1]: ActionView::Template::Error (undefined method `avatar' for nil:NilClass):
2015-05-16T12:17:13.473930+00:00 app[web.1]:     16:       
2015-05-16T12:17:13.473932+00:00 app[web.1]:     17:       <span class="messageuser">
2015-05-16T12:17:13.473933+00:00 app[web.1]:     18:         <%#= conversation.last_message.sender.name %>
2015-05-16T12:17:13.473936+00:00 app[web.1]:     19:         <%= link_to image_tag(conversation.last_message.sender.avatar.url(:mini), class: 'img-rounded'), user_path(conversation.last_message.sender) %>
2015-05-16T12:17:13.473944+00:00 app[web.1]:     20:         <%= link_to conversation.last_message.sender.name, user_path(conversation.last_message.sender) %>
2015-05-16T12:17:13.473946+00:00 app[web.1]:     21:       </span>
2015-05-16T12:17:13.473948+00:00 app[web.1]:     22:       
2015-05-16T12:17:13.473951+00:00 app[web.1]:   app/views/conversations/_conversation.html.erb:19:in `_app_views_conversations__conversation_html_erb___2085506579815819501_69959535266760'
2015-05-16T12:17:13.473952+00:00 app[web.1]:   app/views/conversations/index.html.erb:35:in `_app_views_conversations_index_html_erb___1748297580391940998_69959535151580'
2015-05-16T12:17:13.473954+00:00 app[web.1]: 
2015-05-16T12:17:13.473957+00:00 app[web.1]: 
2015-05-16T12:17:13.572052+00:00 app[web.1]: Started GET "/conversations" for 160.3.94.130 at 2015-05-16 12:17:13 +0000
2015-05-16T12:17:13.579974+00:00 app[web.1]:   User Load (1.3ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1464]]
2015-05-16T12:17:13.585203+00:00 app[web.1]:    (0.9ms)  SELECT COUNT(*) FROM "user_learnings" WHERE "user_learnings"."user_id" = $1  [["user_id", 1464]]
2015-05-16T12:17:13.637517+00:00 app[web.1]:    (3.1ms)  SELECT DISTINCT COUNT(DISTINCT "mailboxer_conversations"."id") FROM "mailboxer_conversations" INNER JOIN "mailboxer_notifications" ON "mailboxer_notifications"."conversation_id" = "mailboxer_conversations"."id" AND "mailboxer_notifications"."type" IN ('Mailboxer::Message') INNER JOIN "mailboxer_receipts" ON "mailboxer_receipts"."notification_id" = "mailboxer_notifications"."id" WHERE "mailboxer_notifications"."type" = 'Mailboxer::Message' AND "mailboxer_receipts"."receiver_id" = $1 AND "mailboxer_receipts"."receiver_type" = $2 AND "mailboxer_receipts"."mailbox_type" = $3 AND "mailboxer_receipts"."trashed" = 'f' AND "mailboxer_receipts"."deleted" = 'f'  [["receiver_id", 1464], ["receiver_type", "User"], ["mailbox_type", "inbox"]]
2015-05-16T12:17:13.678884+00:00 app[web.1]:   Mailboxer::Receipt Load (1.2ms)  SELECT "mailboxer_receipts".* FROM "mailboxer_receipts" WHERE "mailboxer_receipts"."notification_id" = $1  [["notification_id", 1443]]
2015-05-16T12:17:13.698778+00:00 app[web.1]:   User Load (4.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1464]]
2015-05-16T12:17:13.464477+00:00 app[web.1]:   User Load (1.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1937]]

[ACPK]

@suramai - See #339.

class User < ActiveRecord::Base
  acts_as_messageable
  before_destroy { messages.destroy_all }
end