Form linkage doesn't work with edit

[clemnoefr]

• Laravel Version: 5.7
• Laravel-admin: 1.8.18

Description:

I try to use a multiple selection with form linkage (checkbox) in an edit page, when i use operator has it's doesn't work. When i access the page i have an error because i use in_array() with a null value (it's $old in the method ). So how can i resolve my issue please?

PS: it does the same error with the example in the doc in an edit Form but not a create Form

Steps To Reproduce:

Create a Form edit page with a multiple selection form linkage checkbox and use operator has