TriMill

I think the problem you have here is that you aren't giving the plants enough light. Try adding some torches near them.

I think this is caused by the following:

Minecraft might have an array or arraylist of servers (except for LAN) that appear in the server selection menu. When the down arrow is pressed, Minecraft tries to increment the index of the selected server. There is, of course, a check to make sure that you can't go beyond the bounds of the array/arraylist. The bound would be finding the total number of servers on the list, including the LAN servers. So when the last server is selected and the down arrow is pressed, Minecraft increments the index beyond the bounds of the array/arraylist, causing the exception

Source: Just an idea, might be completely wrong.