I am just speculating here..
The error is related to quantity not being set in an array that comes from the data source, this is most likely related to the MSI functionality as it was new in 2.3.
If you are not using MSI with multiple stocks you could start to try to disable it, you can follow the instructions on the devdoc for this:
Otherwise, I think there are some problem in either the data provider for the product_form.xml ui component or the product_form.xml itself. As the error thrown is related to the quantity field you could try to comment out the <field name="quantity" .../> nodes in your product_form.xml and see what happens.
E.g. in this file:
<field name="quantity" component="Magento_InventoryCatalogAdminUi/js/product/form/sources/qty" formElement="input" sortOrder="50">
However, disabling MSI if you are not using it is your best bet.