HotspotManager: set channel to null after closing it consistently

3 jobs for !1515 with 1081-share-app-via-wifi-hotspot in 12 minutes and 11 seconds (queued for 5 seconds)
latest detached
Status Job ID Name Coverage
  Test
passed #11155
test

00:12:11

 
  Optional Tests
manual #11156
kvm allowed to fail manual
android test
manual #11157
allowed to fail manual
bridge test