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async-await-null-race-condition

Fix "Null check operator used on a null value" errors when an object is set to null during an async await. Use when: (1) Object reference is nullified while awaiting, (2) Code accesses object with ! after await returns, (3) Cancel/dispose operations run concurrently with async operations on same object. Solution: capture local reference before await.

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Async Await Null Race Condition

Problem

When an async operation is awaiting, external code can set the object reference to null. When the await completes, accessing the object with ! throws "Null check operator used on a null value".

Context / Trigger Conditions

Error: Null check operator used on a null value
Pattern: await someObject!.asyncMethod() followed by someObject!.property
Object can be nullified by cancel/dispose/cleanup operations
Concurrent operations on shared mutable state
Typically seen with session objects, connection handlers, or service references

Solution

Wrong - Object can become null during await:

Future<Result> waitForResponse() async {
  // _session might be set to null while we're awaiting
  final result = await _session!.waitForConnection();

  if (result == null) {
    // BUG: _session could be null here!
    final state = _session!.state;  // Throws null check error
    ...
  }
}

void cancel() {
  _session?.cancel();
  _session = null;  // This runs while waitForResponse is awaiting
}

Correct - Capture local reference before await:

Future<Result> waitForResponse() async {
  // Capture reference BEFORE await
  final session = _session!;

  final result = await session.waitForConnection();

  // Check if cancelled during await
  if (_session == null) {
    return Result.cancelled();
  }

  if (result == null) {
    // Safe - using captured local reference
    final state = session.state;
    ...
  }
}

Verification

Trigger the cancel/dispose operation while async operation is in progress
No null check errors should occur
Operation should gracefully handle cancellation

Example - Full Pattern

class ConnectionManager {
  Session? _session;

  Future<ConnectionResult> connect() async {
    if (_session == null) {
      return ConnectionResult.failure('No session');
    }

    // Capture before await
    final session = _session!;

    final response = await session.waitForResponse(
      timeout: Duration(minutes: 2),
    );

    // Check if cancelled during await
    if (_session == null) {
      return ConnectionResult.failure('Cancelled');
    }

    // Safe to use captured reference
    if (response == null) {
      return ConnectionResult.failure(session.errorMessage);
    }

    return ConnectionResult.success(response);
  }

  void cancel() {
    _session?.cancel();
    _session?.dispose();
    _session = null;
  }
}

Notes

This pattern applies to any mutable reference that can be nullified externally
Common in Flutter/Dart with dispose patterns, Riverpod providers, and WebSocket sessions
The captured local reference keeps the object alive during the await
Always add a null check after await to detect cancellation
Consider using Completer cancellation patterns for more complex scenarios

Related Patterns

riverpod-ref-read-in-dispose - Similar issue with Riverpod ref lifecycle
flutter-dispose-timer-test-failure - Timer callbacks after disposal

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