Hey @Nikul Having this same issue,
Ves Brand related error.
After installing the Brand extension from LandOfCoder.com
this eror began to occur.
After doing some research and come to the conclusion that because its just a warning its not effecting anything on our backend or our database its just extremely annoying..
So basically when it Calls for is_in_stock its expecting an array but it recieves a string which is in_stock which it does not like so it throws a warning.
If anyone has a better way of explaining it please chime in
To create this error.
Fresh install magento 2.1
mysql 5.7 php7.04 nginx ubuntu 16.04
set developer mode
install Ves_Brand https://ticksy_attachments.s3.amazonaws.com/Ves_Brand_v2.1.0_403.zip
Add brand Group then add product
Result = warning.