Why the midlet has been stoped?

[komomo]

Now I am using a new machine to write MIDlet .
On that machine, whenever I use ALert, the emulator disappears and
displaying the message such as window is trying to stop MIDlet .
Why?
The program is correct.

[ektasrv]

Which development environment / IDE you are using.
In which emulator you are running your app?
Try SUN WTK emulator.