window: Initialize chat with NULL

otherwise 'chat' may point to an invalid memory location when used
7 jobs for master in 16 minutes and 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #342906
librem5
build-pureos-gcc-byzantium

00:03:03

 
  Test
passed #342908
librem5
check-po

00:00:26

passed #342907
librem5
test:debian-gcc

00:02:38

23.2%
 
  Package
passed #342909
librem5
package-pureos-byzantium

00:03:22

passed #342910
librem5:arm64
package-pureos-byzantium:arm64

00:05:54

 
  Test Package
passed #342911
librem5
autopkgtest-pureos-byzantium-package

00:01:32

passed #342912
librem5
lintian-pureos-byzantium-package

00:00:47