The "default NetVM" is usually the first created ProxyVM which is
set by qubes-core during its creation. [1] If there is no ProxyVM,
there is no "default NetVM". Therefore, creating an AppVM and
launching its settings dialog raised AttributeError, because
get_default_netvm method returned None.
This can be reproduced by installing QubesOS without creating VMs
by installer.
[1] https://github.com/QubesOS/qubes-core-admin/blob/master/core/qubes.py#L355Fixesqubesos/qubes-issues#1008