[Reading List iOS] Test WebState on JavaScript return.

ios/chrome/browser/reading_list/reading_list_distiller_page.mm

Index: ios/chrome/browser/reading_list/reading_list_distiller_page.mm
diff --git a/ios/chrome/browser/reading_list/reading_list_distiller_page.mm b/ios/chrome/browser/reading_list/reading_list_distiller_page.mm
index feb04ff68a3039c35627f729ebad3bbe0546f0f0..e2fd850ad8b171d4dc6ff3ffd9f50cfeeff0b21c 100644
--- a/ios/chrome/browser/reading_list/reading_list_distiller_page.mm
+++ b/ios/chrome/browser/reading_list/reading_list_distiller_page.mm
@@ -196,6 +196,11 @@ void ReadingListDistillerPage::DelayedOnLoadURLDone(int delayed_task_id) {
 }
 
 void ReadingListDistillerPage::ContinuePageDistillation() {
+  if (!CurrentWebState()) {
+    // Something interrupted the distillation.
+    // Abort here.
+    return;
+  }
   // The page is ready to be distilled.
   // If the visible URL is not the original URL, notify the caller that URL
   // changed.