alert dialog issue.How to fix it?

E/flutter (25347): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'call' was called on null.
E/flutter (25347): Receiver: null
E/flutter (25347): Tried calling: call()
E/flutter (25347): #0 DialogService.showDialog
package:glowchat/services/dialog_service.dart:26
E/flutter (25347): #1 LoginViewModel.login
package:glowchat/viewmodels/login_view_model.dart:39
E/flutter (25347):
E/flutter (25347): #2 LoginScreen.build..
package:glowchat/…/login/login_screen.dart:126
E/flutter (25347): #3 GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:182
E/flutter (25347): #4 TapGestureRecognizer.handleTapUp
package:flutter/…/gestures/tap.dart:504
E/flutter (25347): #5 BaseTapGestureRecognizer._checkUp
package:flutter/…/gestures/tap.dart:282
E/flutter (25347): #6 BaseTapGestureRecognizer.acceptGesture
package:flutter/…/gestures/tap.dart:254
E/flutter (25347): #7 GestureArenaManager.sweep
package:flutter/…/gestures/arena.dart:156
E/flutter (25347): #8 GestureBinding.handleEvent
package:flutter/…/gestures/binding.dart:222
E/flutter (25347): #9 GestureBinding.dispatchEvent
package:flutter/…/gestures/binding.dart:198
E/flutter (25347): #10 GestureBinding._handlePointerEvent
package:flutter/…/gestures/binding.dart:156
E/flutter (25347): #11 GestureBinding._flushPointerEventQueue
package:flutter/…/gestures/binding.dart:102
E/flutter (25347): #12 GestureBinding._handlePointerDataPacket
package:flutter/…/gestures/binding.dart:86
E/flutter (25347): #13 _rootRunUnary (dart:async/zone.dart:1196:13)
E/flutter (25347): #14 _CustomZone.runUnary (dart:async/zone.dart:1085:19)
E/flutter (25347): #15 _CustomZone.runUnaryGuarded (dart:async/zone.dart:987:7)
E/flutter (25347): #16 _invoke1 (dart:ui/hooks.dart:275:10)
E/flutter (25347): #17 _dispatchPointerDataPacket (dart:ui/hooks.dart:184:5)
E/flutter (25347):