Prevent NPE when trying to compute AddDiagnosticPropertyReference assist inside a mixin

Bug: dart-lang/sdk#42749
Change-Id: I2772f0c0f4f21409730713c1ed8a871725080a2f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154883
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Danny Tuppeny <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/services/correction/dart/add_diagnostic_property_reference.dart

@@ -99,6 +99,9 @@ class AddDiagnosticPropertyReference extends CorrectionProducer {
     }
 
     final classDeclaration = parent.thisOrAncestorOfType<ClassDeclaration>();
+    if (classDeclaration == null) {
+      return;
+    }
     final debugFillProperties =
         classDeclaration.getMethod('debugFillProperties');
     if (debugFillProperties == null) {

pkg/analysis_server/test/src/services/correction/assist/add_diagnostic_property_reference_test.dart

@@ -42,4 +42,14 @@ class W extends Widget {
 }
 ''');
   }
+
+  Future<void> test_notAvailable_mixin() async {
+    verifyNoTestUnitErrors = false;
+    await resolveTestUnit('''
+mixin MyMixin {
+  String get foo/*caret*/() {}
+}
+''');
+    await assertNoAssist();
+  }
 }