Add multi currency demo accounts (#530)

app/models/exchange_rate.rb

@@ -3,6 +3,12 @@ class ExchangeRate < ApplicationRecord
     return amount unless EXCHANGE_RATE_ENABLED
 
     rate = ExchangeRate.find_by(base_currency: from, converted_currency: to)
-    amount * rate.rate
+
+    # TODO: Handle the case where the rate is not found
+    if rate.nil?
+      amount # Silently handle the error by returning the original amount
+    else
+      amount * rate.rate
+    end
   end
 end