If it helps anyone else having this issue, I managed to fix it by doing the following:
1. In the Steam Library page for Divinity: Original Sin 2. Scroll down to the DLC section and ensure "Divinity Engine 2 Data" is checked and installed.
2. The first time I booted the editor, I picked an incorrect Data Path. It cannot be the root Divinity: Original Sin 2 directory. It must be the Data folder inside. This can be changed after the fact by going to steamapps/common/The Divinity Engine 2/Config.xml and changing the URL inside the <DataPath> tags.