fix(performance, ios): fix a crash that would occur when creating a trace with performance collection disabled (#10243)

Author: Lyokone

Diff for: packages/firebase_performance/firebase_performance/example/integration_test/firebase_performance_e2e_test.dart

@@ -3,9 +3,9 @@
 // found in the LICENSE file.
 
 import 'package:firebase_core/firebase_core.dart';
-import 'package:flutter_test/flutter_test.dart';
 import 'package:firebase_performance/firebase_performance.dart';
 import 'package:flutter/foundation.dart' show kIsWeb;
+import 'package:flutter_test/flutter_test.dart';
 import 'package:integration_test/integration_test.dart';
 
 import 'firebase_options.dart';
@@ -46,10 +46,10 @@ void main() {
 
     setUpAll(() async {
       performance = FirebasePerformance.instance;
-      await performance.setPerformanceCollectionEnabled(true);
     });
 
     setUp(() async {
+      await performance.setPerformanceCollectionEnabled(true);
       testTrace = performance.newTrace('test-trace');
     });
 
@@ -58,6 +58,12 @@ void main() {
       await testTrace.stop();
     });
 
+    test('starting trace with performance collection disabled', () async {
+      await performance.setPerformanceCollectionEnabled(false);
+      await testTrace.start();
+      await testTrace.stop();
+    });
+
     test("starting Trace twice shouldn't throw an error", () async {
       await testTrace.start();
       await testTrace.start();

Diff for: packages/firebase_performance/firebase_performance/ios/Classes/FLTFirebasePerformancePlugin.m

@@ -130,6 +130,11 @@ - (void)traceStart:(id)arguments withMethodCallResult:(FLTFirebaseMethodCallResu
   NSString *name = arguments[@"name"];
 
   FIRTrace *trace = [FIRPerformance startTraceWithName:name];
+  if (trace == nil) {
+    // Performance collection is disabled
+    result.success(nil);
+    return;
+  }
   _traceHandle = [NSNumber numberWithInt:[_traceHandle intValue] + 1];
   [_traces setObject:trace forKey:_traceHandle];