[flutter_tools] bail from printing if devtools launch fails (#83934)

packages/flutter_tools/lib/src/resident_devtools_handler.dart

@@ -82,6 +82,10 @@ class FlutterResidentDevtoolsHandler implements ResidentDevtoolsHandler {
       _served = true;
     }
     await _devToolsLauncher.ready;
+    // Do not attempt to print debugger list if the connection has failed.
+    if (_devToolsLauncher.activeDevToolsServer == null) {
+      return;
+    }
     final List<FlutterDevice> devicesWithExtension = await _devicesWithExtensions(flutterDevices);
     await _maybeCallDevToolsUriServiceExtension(devicesWithExtension);
     await _callConnectedVmServiceUriExtension(devicesWithExtension);

packages/flutter_tools/test/general.shard/resident_devtools_handler_test.dart

@@ -192,6 +192,22 @@ void main() {
     );
   });
 
+  testWithoutContext('serveAndAnnounceDevTools will bail if launching devtools fails', () async {
+    final ResidentDevtoolsHandler handler = FlutterResidentDevtoolsHandler(
+      FakeDevtoolsLauncher()..activeDevToolsServer = null,
+      FakeResidentRunner(),
+      BufferLogger.test(),
+    );
+    final FakeVmServiceHost fakeVmServiceHost = FakeVmServiceHost(requests: <VmServiceExpectation>[], httpAddress: Uri.parse('http://localhost:1234'));
+
+    final FakeFlutterDevice device = FakeFlutterDevice()
+      ..vmService = fakeVmServiceHost.vmService;
+
+    await handler.serveAndAnnounceDevTools(
+      flutterDevices: <FlutterDevice>[device],
+    );
+  });
+
   testWithoutContext('serveAndAnnounceDevTools with web device', () async {
     final ResidentDevtoolsHandler handler = FlutterResidentDevtoolsHandler(
       FakeDevtoolsLauncher()..activeDevToolsServer = DevToolsServerAddress('localhost', 8080),

packages/flutter_tools/test/general.shard/resident_runner_test.dart

@@ -1926,7 +1926,6 @@ void main() {
     FileSystem: () => ThrowingForwardingFileSystem(MemoryFileSystem.test()),
   }));
 
-
   testUsingContext('ColdRunner writes vm service file when providing debugging option', () => testbed.run(() async {
     fakeVmServiceHost = FakeVmServiceHost(requests: <VmServiceExpectation>[
       listViews,