Add a small patch that would be part of 3.1b if it gets released.

sysutils/tmux/Makefile

@@ -3,6 +3,7 @@
 
 PORTNAME=	tmux
 PORTVERSION=	3.1a
+PORTREVISION=	1
 CATEGORIES=	sysutils
 MASTER_SITES=	https://github.com/tmux/tmux/releases/download/${PORTVERSION}/
 DISTFILES=	${DISTNAME}${EXTRACT_SUFX}

sysutils/tmux/files/patch-6a33a12798b2afeee6fb7bba74d86d628137921e

@@ -0,0 +1,18 @@
+From 6a33a12798b2afeee6fb7bba74d86d628137921e Mon Sep 17 00:00:00 2001
+From: Nicholas Marriott <nicholas.marriott@gmail.com>
+Date: Thu, 30 Apr 2020 15:20:08 +0100
+Subject: [PATCH] Do not remove the automatic-rename option from the global
+ set, only from the window (it must stay in the global set or tmux will
+ crash). GitHub issue 2188.
+
+--- input.c.orig	2020-04-29 20:01:18 UTC
++++ input.c
+@@ -2304,7 +2304,7 @@ input_exit_rename(struct input_ctx *ictx)
+ 		return;
+ 
+ 	if (ictx->input_len == 0) {
+-		oe = options_get(wp->window->options, "automatic-rename");
++		oe = options_get_only(wp->window->options, "automatic-rename");
+ 		if (oe != NULL)
+ 			options_remove(oe);
+ 		return;