Guard CloudProvider._sendCloudData against a cleared connection

[Chessing234]

Bug

CloudProvider._sendCloudData throws
TypeError: Cannot read properties of null (reading 'send') when a
previously-scheduled cloud-data send fires after the provider has
been cleared (e.g. the user navigates away from a project or
requestCloseConnection() is called while messages are still in
flight).

Root cause

The constructor wraps _sendCloudData in a 100 ms lodash throttle:

this.sendCloudData = throttle(this._sendCloudData, 100);

so a call to sendCloudData(data) can be deferred by up to 100 ms.
In that window the owner can run requestCloseConnection(), which
calls clear():

clear () {
    this.connection = null;
    ...
}

When the throttle's trailing call finally fires it hits
_sendCloudData:

_sendCloudData (data) {
    this.connection.send(`${data}\n`);
}

this.connection is now null, so .send explodes. The call sites
that schedule the send (writeToServer and the onOpen queue flush)
already check this.connection before queueing, but they cannot
speak for what the connection is when the throttle trailing-edge
actually executes.

Why the fix is correct

• Guarding the actual .send with if (this.connection) makes a
  late-arriving throttled call a no-op instead of a crash; messages
  targeted at an already-closed connection were going to be dropped
  anyway (the server can't receive them).
• Every existing code path that has a valid connection continues to
  send exactly as before.
• No behaviour change under normal operation - the guard only fires
  in the post-clear() race window that was previously crashing.

Change

src/lib/cloud-provider.js: wrap this.connection.send(\${data}\n`)in anif (this.connection)` guard.

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