Import assets from speedtree causes crash

Hello everybody,

I currently have some issues trying to export my trees from Speedtree and importing them as assets in unity. The export from Speedtree has no problems but when I try to put the generated files in my unity assets it makes the following thing that is on the image :

And it doesn't go further.
The files I put as assets are these :

Is it crashing because the files are too heavy ? Or do you have any idea that could explain my problem ?

Thank you beforehand,

Martin