Magento 2.3 Add simple product manually to configurable product doesnt work

Our client didn't add simple products to configurable product and then didn't save the configurable product on admin page.   

Step by steps:

1. Go to admin page, edit configurable products 

2. Click "Add Products Manually" 

3. Picked some simple products 

4. Click Done button 

5. Click save button (configurable product could not save)

SOLUTION 1:

I found this solution. Reference link:

https://github.com/magento/magento2/issues/5172

Fixed Patch:

https://github.com/magento/magento2/commit/87b0a4bc1361fb315acb89d5a9f27e98b1bf6b74

SOLUTION 2:

I fixed the problem. In admin, edit product, click Advanced Inventory -> Maximum Qty Allowed in Shopping Cart is set = 0. I changed the value to 9999 and solved the problem.