flutter error on modal bottom sheet Vertical viewport was given unbounded height

i think you should set fixed height of the container here is modified code for you and also i have mentioned that in which line i changed the code ( changed code by me) –

_showModal(BuildContext context) {
  showModalBottomSheet(
    context: context,
    builder: (context) => Container(
      height: 400, // Set a fix height of container , changed code by me
      margin: const EdgeInsets.only(top: 10),
      child: Column(
        children: [
          Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text(
                'Select event',
                style: Theme.of(context).textTheme.headlineMedium,
              ),
            ],
          ),
          Expanded( // changed code by me
            child: ListView.builder(
              physics: const ScrollPhysics(),
              itemCount: widget.user.myEvents!.length,
              itemBuilder: (contex, index) {
                return StreamBuilder(
                  stream: di
                      .gt<GetSingleEventUseCase>()
                      .call(widget.user.myEvents![index]),
                  builder: (context, snapshot) {
                    if (snapshot.hasData) {
                      return TileSettings(
                        title: snapshot.data!.first.eventName!,
                        leading:
                            snapshot.data!.first.guestsid!.contains(widget.otherUser.uid) ||
                                    snapshot.data!.first.guestsid!.contains(widget.otherUser.uid)
                                ? const Image(image: ticketLinear)
                                : const Image(image: ticket),
                        onTap: () {},
                        trailing: const SizedBox.shrink(),
                      );
                    } else {
                      return TileSettings(
                        title: 'No events',
                        leading: const Image(image: cross),
                        onTap: () {},
                        trailing: const SizedBox.shrink(),
                      );
                    }
                  },
                );
              },
            ),
          ),
        ],
      ),
    ),
  );
}

hope this will work, if it works then hit up and mark it as answer so other can take help