Re-using client through Drupal DependencyInjection causes null protocol

[whikloj]

We are using stomp-php with Drupal through the Islandora stack and have hit an issue.

When an attempt to send a message fails, it leaves the StatefulStomp client with an isConnecting state of true.

So if a second message is attempted using the same client we hit

Call to a member function getBeginFrame() on null in Stomp\States\ProducerTransactionState->initTransaction() (line 53 of vendor/stomp-php/stomp-php/src/States/TransactionsTrait.php).

This seems to be because the transaction trait chains getProtocol even though it could be null.

I'd suggest removing the check of isConnecting from the getProtocol function and only rely on isConnected before returning the protocol.
i.e.

public function getProtocol()
{
    if (!$this->isConnected()) {
        $this->connect();
    }
    return $this->protocol;
}

But if there is a good reason for leaving isConnecting in there, then perhaps some method to reset the isConnecting state (as it is a private variable)?