I have done the following to get it working - not sure if this will break something else but seems to be working so far

in the LeadController.php
n the get function
replace:

if ($userIds = bouncer()->getAuthorizedUserIds()) {
    $query->whereIn('leads.user_id', $userIds);
}

with:

$userIds = bouncer()->getAuthorizedUserIds() ?? [];
if (!empty($userIds)) {
    $query->whereIn('leads.user_id', $userIds);
}

and in the view function, replace:
replace

if (
    $userIds = bouncer()->getAuthorizedUserIds()
    && ! in_array($lead->user_id, $userIds)
) {
    return redirect()->route('admin.leads.index');
}

with

$userIds = bouncer()->getAuthorizedUserIds() ?? [];
if (!empty($userIds) && !in_array($lead->user_id, $userIds)) {
    return redirect()->route('admin.leads.index');

I have the same error, too. I'm not sure if it's part of the same problem, but when I try to add a new lead with the non-admin account, I am not able to select the sales owner, so after the lead is created, the person who created it can not see it.

In both cases, I guess it's a permitions problem and they need access to something I missed on that user group.