Completion. Fix for NPE when complete at the start of a DartDoc.

Bug: https://buganizer.corp.google.com/issues/414299463
Change-Id: I70d38a825b5b8eb3d9e3ce0a0dbbe95ea6e5774f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425120
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart

@@ -543,7 +543,11 @@ class TokenData {
             (currentToken.offset == selectionOffset &&
                 !currentToken.isKeywordOrIdentifier)) &&
         !currentToken.isEof) {
-      currentToken = currentToken.previous!;
+      if (currentToken.previous case var previous?) {
+        currentToken = previous;
+      } else {
+        return null;
+      }
     }
     if (currentToken.isEof) {
       return null;

pkg/analysis_server/test/services/completion/dart/location/dart_doc_test.dart

@@ -338,6 +338,18 @@ suggestions
 ''');
   }
 
+  Future<void> test_start() async {
+    // There was a crash because we tried to walk before the (only) token
+    // of the documentation comment.
+    await computeSuggestions('''
+^/// hello
+class A {}
+''');
+    assertResponse(r'''
+suggestions
+''');
+  }
+
   Future<void> test_topLevelGetter() async {
     allowedIdentifiers = const {'myTopLevelGetter'};
     await computeSuggestions('''