X_TEST_ISLAND_STATE_STUB_INVALID
Means
an island route stub is not <METHOD> <pathname>
Typical cause
match is a <METHOD> <pathname> PREFIX; a bare path or a lowercase verb stubs nothing, the component fetches for real, the sealed network refuses it, and the picture shows an error state nobody declared
Fix
write { match: 'GET /api/settings', respond: { kind: 'json', body: {} } }