fix null access for CkParagraph.dispose

lib/web_ui/lib/src/engine/canvaskit/text.dart

@@ -657,7 +657,7 @@ class CkParagraph extends SkiaObject<SkParagraph> implements ui.Paragraph {
 
   @override
   void delete() {
-    _skParagraph!.delete();
+    _skParagraph?.delete();
   }
 
   @override

lib/web_ui/test/canvaskit/canvaskit_api_test.dart

@@ -1342,6 +1342,18 @@ void _canvasTests() {
     );
   });
 
+  test('Paragraph dispose', () {
+    final CkParagraphBuilder builder = CkParagraphBuilder(
+      CkParagraphStyle(),
+    );
+    builder.addText('Hello');
+    final CkParagraph paragraph = builder.build();
+
+    paragraph.delete();
+    paragraph.dispose();
+    expect(paragraph.debugDisposed, true);
+  });
+
   test('toImage.toByteData', () async {
     // Pretend that FinalizationRegistry is supported, so we can run this
     // test in older browsers (the test will use a TestCollector instead of