This should do it.

app/models/warning_template.rb

@@ -6,12 +6,12 @@ class WarningTemplate < ApplicationRecord
   def body_as_b64
     body_with_site_replacements = body.gsub '$SiteName', SiteSetting['SiteName']
 
-    body_with_site_replacements = if SiteSetting['ChatLink'].nil?
-      body_with_site_replacements.gsub '$ChatLink', 'chat'
+    chat_link = if SiteSetting['ChatLink'].nil?
+                  'chat'
                 else
-      chat_link = '[chat](' + SiteSetting['ChatLink'] + ')'
-      body_with_site_replacements.gsub '$ChatLink', chat_link
+                  '[chat](' + SiteSetting['ChatLink'] + ')'
                 end
+    body_with_site_replacements = body_with_site_replacements.gsub '$ChatLink', chat_link
 
     Base64.encode64(body_with_site_replacements)
   end