Add fix for issue botman#1305

Author: filippotoso

src/Traits/HandlesConversations.php

@@ -107,7 +107,11 @@ public function removeStoredConversation($message = null)
          * Only remove it from the cache if it was not modified
          * after we loaded the data from the cache.
          */
-        if ($this->getStoredConversation($message)['time'] == $this->currentConversationData['time']) {
+
+        $conversation = $this->getStoredConversation($message);
+
+        // See https://github.com/botman/botman/issues/1305
+        if (isset($conversation['time']) && ($conversation['time'] == $this->currentConversationData['time'])) {
             $this->cache->pull($this->message->getConversationIdentifier());
             $this->cache->pull($this->message->getOriginatedConversationIdentifier());
         }