Fallback function has to be external: backwards-compatible changes.

docs/security-considerations.rst

@@ -213,7 +213,7 @@ Now someone tricks you into sending ether to the address of this attack wallet:
             owner = msg.sender;
         }
 
-        function() public {
+        function() external {
             TxUserWallet(msg.sender).transferTo(owner, msg.sender.balance);
         }
     }