After some investigation :
Combobox:clear() don't work as expected (it don't clear at all the combobox). It seems that this bug is very old and have never arised since your report.
Combobox:remove() don't check for wrong value type (for example, can remove first item if
nil is provided)
Combobox:add() seems to work as expected, can you provide a sample that repoduce this ?
I will publish a fix very soon. Thanks for your help !