[ Widget Preview ] Null assertion failure when adding @Preview() to unsupported AST nodes #173959
Description
flutter widget-preview start will currently crash on 3.35.{0,1} if the @Preview() annotation is added to an AST node that isn't part of a valid preview.
For example, adding @Preview() to a class declaration will cause the following crash:
Thread 0 main thread_TypeError: Null check operator used on a null value
at _PreviewVisitor.visitAnnotation(dependency_graph.dart:108)
at AnnotationImpl.accept(ast.dart:366)
at NodeListImpl.accept(ast.dart:13243)
at _AnnotatedNodeMixin._visitCommentAndAnnotations(ast.dart:19496)
at AnnotatedNodeImpl.visitChildren(ast.dart:165)
at ClassDeclarationImpl.visitChildren(ast.dart:2939)
at RecursiveAstVisitor.visitClassDeclaration(visitor.dart:940)
at ClassDeclarationImpl.accept(ast.dart:2935)
at NodeListImpl.accept(ast.dart:13243)
at CompilationUnitImpl.visitChildren(ast.dart:3611)
at LibraryPreviewNode.findPreviews(dependency_graph.dart:185)
at PreviewDetector._findPreviewFunctions(preview_detector.dart:202)
at <asynchronous gap>(async)
at PreviewDetector.initialize(preview_detector.dart:54)
at <asynchronous gap>(async)
at WidgetPreviewStartCommand.runCommand(widget_preview.dart:305)
at <asynchronous gap>(async)
at FlutterCommand.run.<anonymous closure>(flutter_command.dart:1581)
at <asynchronous gap>(async)
at AppContext.run.<anonymous closure>(context.dart:154)
at <asynchronous gap>(async)
at CommandRunner.runCommand(command_runner.dart:212)
at <asynchronous gap>(async)
at FlutterCommandRunner.runCommand.<anonymous closure>(flutter_command_runner.dart:503)
at <asynchronous gap>(async)
at AppContext.run.<anonymous closure>(context.dart:154)
at <asynchronous gap>(async)
at FlutterCommandRunner.runCommand(flutter_command_runner.dart:438)
at <asynchronous gap>(async)
at run.<anonymous closure>.<anonymous closure>(runner.dart:98)
at <asynchronous gap>(async)
at AppContext.run.<anonymous closure>(context.dart:154)
at <asynchronous gap>(async)
at main(executable.dart:101)
at <asynchronous gap>(async)
Annotated nodes that aren't valid previews should be ignored.
This is a top 10 crasher for the tool for 3.35.{0,1}.