iOS: add null check on create impeller context

shell/platform/darwin/graphics/FlutterDarwinContextMetalImpeller.mm

@@ -25,14 +25,8 @@
       std::make_shared<fml::NonOwnedMapping>(impeller_framebuffer_blend_shaders_data,
                                              impeller_framebuffer_blend_shaders_length),
   };
-  auto context = impeller::ContextMTL::Create(shader_mappings, is_gpu_disabled_sync_switch,
-                                              "Impeller Library");
-  if (!context) {
-    FML_LOG(ERROR) << "Could not create Metal Impeller Context.";
-    return nullptr;
-  }
-
-  return context;
+  return impeller::ContextMTL::Create(shader_mappings, is_gpu_disabled_sync_switch,
+                                      "Impeller Library");
 }
 
 @implementation FlutterDarwinContextMetalImpeller
@@ -41,11 +35,9 @@ - (instancetype)init:(const std::shared_ptr<const fml::SyncSwitch>&)is_gpu_disab
   self = [super init];
   if (self != nil) {
     _context = CreateImpellerContext(is_gpu_disabled_sync_switch);
+    FML_CHECK(_context) << "Could not create Metal Impeller Context.";
     id<MTLDevice> device = _context->GetMTLDevice();
-    if (!device) {
-      FML_DLOG(ERROR) << "Could not acquire Metal device.";
-      return nil;
-    }
+    FML_CHECK(device) << "Could not acquire Metal device.";
 
     CVMetalTextureCacheRef textureCache;
     CVReturn cvReturn = CVMetalTextureCacheCreate(kCFAllocatorDefault,  // allocator
@@ -55,10 +47,7 @@ - (instancetype)init:(const std::shared_ptr<const fml::SyncSwitch>&)is_gpu_disab
                                                   &textureCache  // [out] cache
     );
 
-    if (cvReturn != kCVReturnSuccess) {
-      FML_DLOG(ERROR) << "Could not create Metal texture cache.";
-      return nil;
-    }
+    FML_CHECK(cvReturn == kCVReturnSuccess) << "Could not acquire Metal device.";
     _textureCache.Reset(textureCache);
   }
   return self;