`gtk_at_spi_cache_add_context()` checks if the GtkAtSpiContext's path
is NULL before inserting the context object into the hash table.
Do the same in `gtk_at_spi_cache_remove_context()` to avoid a NULL
pointer dereference in `g_str_hash()` during the hash table lookup
if a context with NULL path is removed. That can happen when the
GtkAtSpiRoot::base_path is NULL, which, in turn, can happen if
`g_application_get_dbus_object_path()` returns NULL.
==394047==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fd1966f8b84 bp 0x7fff11e3ded0 sp 0x7fff11e3de58 T0)
==394047==The signal is caused by a READ memory access.
==394047==Hint: address points to the zero page.
#0 0x7fd1966f8b84 in g_str_hash (/usr/lib/libglib-2.0.so.0+0x37b84)
#1 0x7fd1966f9c09 in g_hash_table_contains (/usr/lib/libglib-2.0.so.0+0x38c09)
#2 0x7fd196062c10 in gtk_at_spi_cache_remove_context ../gtk/a11y/gtkatspicache.c:447
#3 0x7fd19606e0a9 in gtk_at_spi_root_unregister ../gtk/a11y/gtkatspiroot.c:653
#4 0x7fd196067f58 in gtk_at_spi_context_unrealize ../gtk/a11y/gtkatspicontext.c:1559
#5 0x7fd195ced97f in gtk_at_context_unrealize ../gtk/gtkatcontext.c:668
#6 0x7fd195f5576e in gtk_widget_unroot_at_context ../gtk/gtkwidget.c:2399
#7 0x7fd195f55bd2 in gtk_widget_unroot ../gtk/gtkwidget.c:2499
...
If the accessible object is hidden, we can skip the emission of the
AddAccessible and RemoveAccessible signals on the cache, as those
objects won't be visible in the accessibility tree.
The GtkAtSpiContext is responsible for removing itself from the root,
which will remove itself from the cache. Any code path that leads to the
GtkAtSpiContext instance being collected passes through the
unrealization phase, which will also unregister the context from the
accessibility bus and from the cache.
The AT-SPI cache interface is used to quickly populate the accessible
objects tree.
The tricky bit is ensuring that we emit change notifications on the
cache only when the cache is available, which means waiting until the
root is asynchronously registered.
