Check your PHP versions (CLI and Server) are compatible with the Magento 2.3 requirements
See the docs for the system requirements: https://devdocs.magento.com/guides/v2.3/install-gde/system-requirements-tech.html#php
The following PHP versions were specified in another thread for the same/similar issue on Stackoverflow
For Magento 2.3 : PHP 7.1
For Magento 2.2 : PHP 7.1 or PHP 7.0
For Magento 2.1 : PHP 7.1, PHP 7.0 or PHP 5.6
For Magento 2.0 : PHP 7.0, PHP 5.6 or PHP 5.5
Link to the StackOverflow thread: https://stackoverflow.com/questions/54867654/uncaught-typeerror-return-value-of-magento-inventorycomposerinstaller-must-be-a
