fix: use sentinel instead of null in storyQueue to avoid NPE

LinkedBlockingQueue.put(null) throws NullPointerException. Replace the
nullable queue with a String queue and map null to/from a DONE_SENTINEL
so callers can still use pushStory(null) to signal completion.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/rn-storybook-auto-screenshots/android/src/main/java/com/rnstorybookautoscreenshots/StorybookRegistry.kt

@@ -34,7 +34,9 @@ class StorybookRegistry(reactContext: ReactApplicationContext) : ReactContextBas
 
         // Capacity 1 so pushStory() blocks until JS has consumed the current entry,
         // giving natural back-pressure between the test runner and JS.
-        private val storyQueue = LinkedBlockingQueue<String?>(1)
+        // LinkedBlockingQueue does not allow null, so we use a sentinel to signal done.
+        private const val DONE_SENTINEL = "__done__"
+        private val storyQueue = LinkedBlockingQueue<String>(1)
 
         @Volatile private var storyReadyLatch: CountDownLatch? = null
 
@@ -45,7 +47,7 @@ class StorybookRegistry(reactContext: ReactApplicationContext) : ReactContextBas
          * Blocks until JS has consumed the previous entry (queue capacity = 1).
          */
         fun pushStory(storyId: String?) {
-            storyQueue.put(storyId)
+            storyQueue.put(storyId ?: DONE_SENTINEL)
         }
 
         /**
@@ -103,8 +105,8 @@ class StorybookRegistry(reactContext: ReactApplicationContext) : ReactContextBas
     @ReactMethod
     fun awaitNextStory(promise: Promise) {
         Thread {
-            val storyId = storyQueue.poll(30, TimeUnit.SECONDS)
-            promise.resolve(storyId)
+            val raw = storyQueue.poll(30, TimeUnit.SECONDS)
+            promise.resolve(if (raw == DONE_SENTINEL) null else raw)
         }.start()
     }