I see many people have been having problems building maps (with speedtrees on them) without the editor crashing; so I figured I would make a quick tutorial for it.
First, load your tree onto your map and make sure it is selected.
Then Right click and select SpeedTreeActor Properties.
Next, scroll down and open SpeedTreeActor followed by Speed Tree Compone.
Now, Scroll down a little more and select SpeedTree and the look for Lod 3DEnd.
Now don't worry if your values aren't the same as mine, that depends on the settings you chose when you made your tree. What we are going to do, is change them so the editor can handle them. Change your values to the ones below.
Now, exit out of properties, save and build. You should have built successfully and should see your tree when you enter the game.
If you are still crashing, try lowering the values a little more and try again.
Hope this helps!