This is JDK issue in macOS 10.15 beta
https://bugs.openjdk.java.net/browse/JDK-8227836
We will check what we can do with that.
At the moment workaround is using of the bob.jar for bundling on macOS 10.15 beta.
UPD: The issue for a future tracking FileDialog doesn't work · Issue #2688 · defold/editor2-issues · GitHub