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[英]flutter: There should be exactly one item with [DropdownButton]'s value: A. Either zero or 2 or more [DropdownMenuItem]s were detected with the same
我用 Flutter 制作了 DropdownButton,但出現錯誤“應該只有一個項目具有 [DropdownButton] 的值:A。檢測到零個或 2 個或更多 [DropdownMenuItem] 具有相同的值 'package:flutter /src/material/dropdown.d ...