How Can I Fix this problem “An error occurred while moving messages to mailbox “(null)”.

If you are having an issue with moving emails to the Trash, I would suggest rebuilding the affected mailboxes, in this case, both the source (where the message was when you tried to delete it) and the Trash. You can try to rebuild mailboxes to resolve “An error occurred while moving messages to mailbox “(null)” by following the steps:
step 1: Firstly, to Rebuild a mailbox.
Step 2: Select a Mailbox, then choose Mailbox > Rebuild.
Step 3: When you Rebuild Mailboxes for IMAP and Exchange accounts then messages and attachments are downloaded again from the server to your Mac. Your mailbox appears empty until the download is complete.

You can also Try this Method.
Step 1: First of all, close all the applications. Then, launch the Apple Mail program.
Step 2:On the Task bar, click Mail tab >> Quit Mail
Step 3:Now hold the Shift key and launch Apple Mail program again. Try the safe mode if Mac doesn’t end starting up
Step 4:Check how it responds in the safe mode.
Step 5:If you find no more issues, you can now reboot it normally.
Step 6:Finally, try testing in your own user’s account.

I experienced the same error “An error occurred while moving messages to mailbox“(null)”.. I rebuilt the mailbox but that did not solve the problem. I deleted the account and set up again. No problems after that. Maybe it will also work for you.

I found a way to fix this. I am assuming you’re using an IMAP account.
Step 1: Firstly Expand the mail account
step 2: Select the folder that you want to use for Trash
step 3: GOTO Mailbox menu>> Trash Mailbox.
You can do the same for sent Messages. Once I did this, the message stopped appearing.