This happened for my projectmanager case, too.
However, it first occurred right after upgrading from 2.9.7 to 3.0.3 (including the database migration thereafter).
The code works now, after I manually edited lib/core.php around line 932, where the error was caused.
In my edited version, that loop now looks like this:
foreach ( $meta AS $m ) {
$key = sanitize_title($m->label);
if($key != null && $key != "")
{$dataset->{$key} = $m->value;}
}
So I just added the "if" conditions.
The php interpreter cannot again be stuck in the assignment with an empty property.
Time for another Bugfix? Or my issue only?