Is It Possible to Migrate a Map That Contains a Tiled Landscape?

When I try and import a 8129x8129 tiled landscape into my 4.21 company project it crashes every time i hit ‘import’.

If i start an entirely new 4.21 project it works perfectly every time!

i have no idea why I can’t add it to my company project, so was wondering if it was possible to migrate a tiled map from a fresh project into my company project instead?

Any help greatly appreciated!
J