Flutter the following _CastError was thrown while handling a gesture

You may see this error

======== Exception caught by gesture ===============================================================
The following _CastError was thrown while handling a gesture:
Null check operator used on a null value

When the exception was thrown, this was the stack: 
#0      ExtensionDialog.dialog (package:get/get_navigation/src/extension_navigation.dart:81:54)
#1      _MapScreenState.build.<anonymous closure>.<anonymous closure> (package:g_map/map_screen.dart:48:36)
#2      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:198:24)
#3      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:608:11)
#4      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:296:5)
#5      BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:230:7)
#6      PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:563:9)
#7      PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:94:12)
#8      PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:139:9)
#9      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:539:8)
#10     PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:137:18)
#11     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:123:7)
#12     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:439:19)
#13     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:419:22)
#14     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:322:11)
#15     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:374:7)
#16     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:338:5)
#17     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:296:7)
#18     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:279:7)
#22     _invoke1 (dart:ui/hooks.dart:170:10)
#23     PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:331:7)
#24     _dispatchPointerDataPacket (dart:ui/hooks.dart:94:31)
(elided 3 frames from dart:async)
Handler: "onTap"
Recognizer: TapGestureRecognizer#788e4
  debugOwner: GestureDetector
  state: possible
  won arena
  finalPosition: Offset(115.0, 319.0)
  finalLocalPosition: Offset(105.0, 16.0)
  button: 1
  sent tap down

The above error happens when you try to use Get.dialogue() from Getx package. Specially when you try. to use onTap() and Get.dialogue() together.

Reason

You did not wrap your app using GetMaterialApp. See the code below

class _MyAppState extends State<MyApp> {

  @override
  Widget build(BuildContext context) {
    Get.put(LocationController());
    return  const GetMaterialApp(
      debugShowCheckedModeBanner: false,
        home: MapScreen()
    );
  }
}

Learn how to use Get.snackbar()