Fix crash on processing devtools event (#1875)

* Fix chrash on processing devtools event

* Address CR comments

Author: Anna Gringauze

dwds/CHANGELOG.md

@@ -14,6 +14,8 @@
 - Pre-warm expression compiler cache to speed up Flutter Inspector loading.
 - Remove `ChromeProxyService.setExceptionPauseMode()`.
 - Display full error on failure to start DDS.
+- Fix crash on processing DevTools event when starting DevTools from observatory
+  uri.
 
 ## 16.0.1
 

dwds/lib/src/dwds_vm_client.dart

@@ -153,29 +153,35 @@ void _processSendEvent(Map<String, dynamic> event,
         final action = payload?['action'] as String?;
         final screen = payload?['screen'] as String?;
         if (screen != null && action == 'pageReady') {
-          if (dwdsStats.isFirstDebuggerReady) {
-            final debuggerReadyTime = DateTime.now()
-                .difference(dwdsStats.devToolsStart)
-                .inMilliseconds;
-            emitEvent(DwdsEvent.devToolsLoad(debuggerReadyTime, screen));
-            _logger.fine('DevTools load time: $debuggerReadyTime ms');
-            final debuggerStartTime = DateTime.now()
-                .difference(dwdsStats.debuggerStart)
-                .inMilliseconds;
-            emitEvent(DwdsEvent.debuggerReady(debuggerStartTime, screen));
-            _logger.fine('Debugger ready time: $debuggerStartTime ms');
-          } else {
-            _logger
-                .finest('Debugger and DevTools startup times already recorded.'
-                    ' Ignoring $event.');
-          }
+          _recordDwdsStats(dwdsStats, screen);
         } else {
           _logger.finest('Ignoring unknown event: $event');
         }
       }
   }
 }
 
+void _recordDwdsStats(DwdsStats dwdsStats, String screen) {
+  if (dwdsStats.isFirstDebuggerReady) {
+    final devToolsStart = dwdsStats.devToolsStart;
+    final debuggerStart = dwdsStats.debuggerStart;
+    if (devToolsStart != null) {
+      final devToolLoadTime =
+          DateTime.now().difference(devToolsStart).inMilliseconds;
+      emitEvent(DwdsEvent.devToolsLoad(devToolLoadTime, screen));
+      _logger.fine('DevTools load time: $devToolLoadTime ms');
+    }
+    if (debuggerStart != null) {
+      final debuggerReadyTime =
+          DateTime.now().difference(debuggerStart).inMilliseconds;
+      emitEvent(DwdsEvent.debuggerReady(debuggerReadyTime, screen));
+      _logger.fine('Debugger ready time: $debuggerReadyTime ms');
+    }
+  } else {
+    _logger.finest('Debugger and DevTools stats are already recorded.');
+  }
+}
+
 Future<Map<String, dynamic>> _hotRestart(
     ChromeProxyService chromeProxyService, VmService client) async {
   _logger.info('Attempting a hot restart');

dwds/lib/src/events.dart

@@ -8,12 +8,12 @@ import 'package:vm_service/vm_service.dart';
 
 class DwdsStats {
   /// The time when the user starts the debugger.
-  late DateTime _debuggerStart;
-  DateTime get debuggerStart => _debuggerStart;
+  DateTime? _debuggerStart;
+  DateTime? get debuggerStart => _debuggerStart;
 
   /// The time when dwds launches DevTools.
-  late DateTime _devToolsStart;
-  DateTime get devToolsStart => _devToolsStart;
+  DateTime? _devToolsStart;
+  DateTime? get devToolsStart => _devToolsStart;
 
   /// Records and returns weither the debugger is ready.
   bool _isFirstDebuggerReady = true;