Do not assume imageCache is created when handleMemoryPressure is called (#58503)

* Do not assume imageCache is created when handleMemoryPressure is called

* comment for test

packages/flutter/lib/src/painting/binding.dart

@@ -102,7 +102,7 @@ mixin PaintingBinding on BindingBase, ServicesBinding {
   @override
   void handleMemoryPressure() {
     super.handleMemoryPressure();
-    imageCache.clear();
+    imageCache?.clear();
   }
 
   /// Listenable that notifies when the available fonts on the system have

packages/flutter/test/painting/image_cache_binding_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/painting.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:flutter/services.dart';
+
+import 'package:flutter_test/flutter_test.dart';
+
+
+void main() {
+  test('PaintingBinding with memory pressure before initInstances', () {
+    // Observed in devicelab: the device sends a memory pressure event to us
+    // after PaintingBinding has been created but before initInstances called,
+    // meaning the imageCache member is still null.
+    final PaintingBinding binding = TestPaintingBinding();
+    expect(binding.imageCache, null);
+    binding.handleMemoryPressure();
+    expect(binding.imageCache, null);
+    binding.initInstances();
+    expect(binding.imageCache, isNotNull);
+    expect(binding.imageCache.currentSize, 0);
+  });
+}
+
+class TestBindingBase implements BindingBase {
+  @override
+  void initInstances() {}
+
+  @override
+  void initServiceExtensions() {}
+
+  @override
+  Future<void> lockEvents(Future<void> Function() callback) async {}
+
+  @override
+  bool get locked => throw UnimplementedError();
+
+  @override
+  Future<void> performReassemble() {
+    throw UnimplementedError();
+  }
+
+  @override
+  void postEvent(String eventKind, Map<String, dynamic> eventData) {}
+
+  @override
+  Future<void> reassembleApplication() {
+    throw UnimplementedError();
+  }
+
+  @override
+  void registerBoolServiceExtension({String name, AsyncValueGetter<bool> getter, AsyncValueSetter<bool> setter}) {}
+
+  @override
+  void registerNumericServiceExtension({String name, AsyncValueGetter<double> getter, AsyncValueSetter<double> setter}) {}
+
+  @override
+  void registerServiceExtension({String name, ServiceExtensionCallback callback}) {}
+
+  @override
+  void registerSignalServiceExtension({String name, AsyncCallback callback}) {}
+
+  @override
+  void registerStringServiceExtension({String name, AsyncValueGetter<String> getter, AsyncValueSetter<String> setter}) {}
+
+  @override
+  void unlocked() {}
+
+  @override
+  ui.Window get window => TestWindow(window: ui.window);
+}
+
+class TestPaintingBinding extends TestBindingBase with SchedulerBinding, ServicesBinding, PaintingBinding {}