Exception caused by tx_list not defined

I wasn't seeing this before, so I assume it could be a race condition if
the "load_wallet" function doesn't finish before the history tab is updated.

plugins/exchange_rate.py

@@ -343,7 +343,10 @@ class Plugin(BasePlugin):
 
     def history_tab_update(self):
         if self.config.get('history_rates', 'unchecked') == "checked":
-            tx_list = self.tx_list
+            try:
+                tx_list = self.tx_list
+            except Exception:
+                return
 
             try:
                 mintimestr = datetime.datetime.fromtimestamp(int(min(tx_list.items(), key=lambda x: x[1]['timestamp'])[1]['timestamp'])).strftime('%Y-%m-%d')