The reason behind the error is says both material/bottom_sheet.dart and bottom_sheet_route exports the ModalBottomSheetRoute.
'ModalBottomSheetRoute' is imported from both
'package:flutter/src/material/bottom_sheet.dart' and 'package:modal_bottom_sheet/src/bottom_sheet_route.dart'.
In order to fix this issue we have to hide one of the ModalBottomSheetRoute. since we need this to be imported from bottom_sheet_route we need to hide it from material
This is the way that we can fix,
Relace
import 'package:flutter/material.dart' with
import 'package:flutter/material.dart' hide ModalBottomSheetRoute;
in the following files.
/Users/<usename>/.pub-cache/hosted/pub.dev/modal_bottom_sheet-2.1.2/lib/src/material_with_modal_page_route.dart/Users/<usename>/.pub-cache/hosted/pub.dev/modal_bottom_sheet-2.1.2/lib/src/bottom_sheets/bar_bottom_sheet.dart/Users/<usename>/.pub-cache/hosted/pub.dev/modal_bottom_sheet-2.1.2/lib/src/bottom_sheets/material_bottom_sheet.dart