window: Initialize chat with NULL

otherwise 'chat' may point to an invalid memory location when used
7 jobs for wip/sadiq/fix-invalid-memory in 12 minutes and 35 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #342890
librem5
build-pureos-gcc-byzantium

00:03:13

 
  Test
passed #342892
librem5
check-po

00:00:28

passed #342891
librem5
test:debian-gcc

00:02:45

23.2%
 
  Package
passed #342893
librem5
package-pureos-byzantium

00:03:13

passed #342894
librem5:arm64
package-pureos-byzantium:arm64

00:05:59

 
  Test Package
passed #342895
librem5
autopkgtest-pureos-byzantium-package

00:01:48

passed #342896
librem5
lintian-pureos-byzantium-package

00:01:02