Fix error when reassign issue/merge_request from non-assigned. Fixes #3201

app/mailers/emails/issues.rb

@@ -8,7 +8,7 @@ module Emails
     def reassigned_issue_email(recipient_id, issue_id, previous_assignee_id)
       @issue = Issue.find(issue_id)
-      @previous_assignee ||= User.find(previous_assignee_id)
+      @previous_assignee = User.find_by_id(previous_assignee_id) if previous_assignee_id
       @project = @issue.project
       mail(to: recipient(recipient_id), subject: subject("changed issue ##{@issue.id}", @issue.title))
     end

app/mailers/emails/merge_requests.rb

@@ -8,7 +8,7 @@ module Emails
     def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_id)
       @merge_request = MergeRequest.find(merge_request_id)
-      @previous_assignee ||= User.find(previous_assignee_id)
+      @previous_assignee = User.find_by_id(previous_assignee_id) if previous_assignee_id
       @project = @merge_request.project
       mail(to: recipient(recipient_id), subject: subject("changed merge request !#{@merge_request.id}", @merge_request.title))
     end

app/views/notify/reassigned_issue_email.html.haml

@@ -2,8 +2,10 @@
   = "Reassigned Issue ##{@issue.id}"
   = link_to_gfm truncate(@issue.title, length: 30), project_issue_url(@issue.project, @issue)
 %p
-  Assignee changed from
-  %strong #{@previous_assignee.name}
+  Assignee changed
+  - if @previous_assignee
+    from
+    %strong #{@previous_assignee.name}
   to
   %strong #{@issue.assignee_name}

app/views/notify/reassigned_issue_email.text.erb

@@ -2,6 +2,4 @@ Reassigned Issue <%= @issue.id %>
 <%= url_for(project_issue_url(@issue.project, @issue)) %>
           
-
-Assignee changed from <%= @previous_assignee.name %> to <%= @issue.assignee_name %>
-
+Assignee changed <%= "from #{@previous_assignee.name}" if @previous_assignee %> to <%= @issue.assignee_name %>

app/views/notify/reassigned_merge_request_email.html.haml

@@ -2,7 +2,9 @@
   = "Reassigned Merge Request !#{@merge_request.id}"
   = link_to_gfm truncate(@merge_request.title, length: 30), project_merge_request_url(@merge_request.project, @merge_request)
 %p
-  Assignee changed from
-  %strong #{@previous_assignee.name}
+  Assignee changed
+  - if @previous_assignee
+    from
+    %strong #{@previous_assignee.name}
   to
   %strong #{@merge_request.assignee_name}

app/views/notify/reassigned_merge_request_email.text.erb

@@ -3,5 +3,5 @@ Reassigned Merge Request <%= @merge_request.id %>
 <%= url_for(project_merge_request_url(@merge_request.project, @merge_request)) %>
           
-Assignee changed from <%= @previous_assignee.name %> to <%= @merge_request.assignee_name %>
+Assignee changed <%= "from #{@previous_assignee.name}" if @previous_assignee %> to <%= @merge_request.assignee_name %>