[flutter_tools] Fallback discovery: Don't crash when an Isolate has no root library (#52354)

packages/flutter_tools/lib/src/ios/fallback_discovery.dart

@@ -167,7 +167,7 @@ class FallbackDiscovery {
             throw Exception('Expected Isolate but found Sentinel: $isolateResponse');
           }
           final LibraryRef library = (isolateResponse as Isolate).rootLib;
-          if (library.uri.startsWith('package:$packageName')) {
+          if (library != null && library.uri.startsWith('package:$packageName')) {
             UsageEvent(
               _kEventName,
               'success',

packages/flutter_tools/test/general.shard/ios/fallback_discovery_test.dart

@@ -67,6 +67,31 @@ void main() {
     ), Uri.parse('http://localhost:1'));
   });
 
+  testUsingContext('Selects assumed port when another isolate has no root library', () async {
+    when(mockVmService.getVM()).thenAnswer((Invocation invocation) async {
+      return VM()..isolates = <IsolateRef>[
+        IsolateRef()..id = '1',
+        IsolateRef()..id = '2',
+      ];
+    });
+    when(mockVmService.getIsolate('1')).thenAnswer((Invocation invocation) async {
+      return Isolate()
+        ..rootLib = null;
+    });
+    when(mockVmService.getIsolate('2')).thenAnswer((Invocation invocation) async {
+      return Isolate()
+        ..rootLib = (LibraryRef()..uri = 'package:hello/main.dart');
+    });
+    expect(await fallbackDiscovery.discover(
+      assumedDevicePort: 23,
+      deivce: null,
+      hostVmservicePort: 1,
+      packageId: null,
+      usesIpv6: false,
+      packageName: 'hello',
+    ), Uri.parse('http://localhost:1'));
+  });
+
   testUsingContext('Selects mdns discovery if VM service connecton fails due to Sentinel', () async {
     when(mockVmService.getVM()).thenAnswer((Invocation invocation) async {
       return VM()..isolates = <IsolateRef>[