There appears to be a slight inconsistency between how g_simple_async_result_new  and g_simple_async_report_error_in_idle work. The former allows the first parameter, a GObject, to be NULL, while the latter does not. Additionally, g_simple_async_report_take_gerror_in_idle contains the same bug.

Change is as simple as modifying:

g_return_if_fail (G_IS_OBJECT (object));

to

g_return_val_if_fail (!object || G_IS_OBJECT (object), NULL);

for both g_simple_async_report_take_gerror_in_idle and g_simple_async_report_error_in_idle