I have this error

[ninjai001]

package:flutter/src/widgets/media_query.dart': Failed assertion: line 819 pos 12: 'context != null': is not true
c

[function1983]

Try passing in BuildContext of the parent widget

GestureDetector(
    key: btnKey,
    onTap: () => customBackground(btnKey, context),
    child: Container()
)

void customBackground(GlobalKey btnKey, BuildContext mainContext) {
    menu.dismiss();
    menu = PopupMenu(
      context: mainContext,
      items: [
          MenuItem(
              title: 'Mail',
              image: Icon(
                Icons.mail,
                color: Colors.white,
              )),
        ],
        onClickMenu: onClickMenu,
        stateChanged: stateChanged,
        onDismiss: onDismiss);
    menu.show(widgetKey: btnKey);
}