X_ISLAND_INVALID
Means
an island declaration cannot become a client entry
Typical cause
no src, a remote URL, a file not named *.island.tsx, unsafe characters, or two islands on one page claiming the same id. The extension is the rule: a module ships to the browser iff its name says so, so the bundler decides without opening the file
Fix
rename the module to <name>.island.tsx and point src at it — the cause names the offending value