Warning: foreach() argument must be of type array in Element\Checkboxes

please refer to attached patch.

Nope, that didn't solve. Now I have the same warning above once and this new one repeated 3 times:

Warning: Array to string conversion in Drupal\Core\Render\Element\Checkbox::processCheckbox() (line 129 of core/lib/Drupal/Core/Render/Element/Checkbox.php).

New warning (line 129 of ...) repeats 4 times in my project after #2 patch.

Thanks @shivam. MR !5 worked for me.
+1 for RTBC

I have reproduced this issue and MR-5 successfully applied for me and fixed the issue for me.
Before:

No warning after this patch, the code looks good to me thus moving this to RTBC+!.