It seems that without explicit initialization of the members they remain
uninitialized. The lack of initialization caused crashing, because
PavuApplication::on_activate() creates the MainWindow only if mainWindow
is NULL, and if it's some random value instead, on_activate() calls
selectTab() on a non-existent window.
Fixes: https://gitlab.freedesktop.org/pulseaudio/pavucontrol/issues/49
d3b3bee378