Cherry-pick MR for !438.
If the plugin's main class is instantiated and destroyed in shortsuccession, its async QCoro functions should not continue executingafter getting their async D-Bus call results.
This commit checks for object aliveness after all awaited coroutines,which should fix async-related crashes in libbrightnesscontrolplugin.
Fixes plasma-mobile#404 properly.
(cherry picked from commit 63aa1ba8)
Co-authored-by: Jakob Petsovits jpetso@petsovits.com