Go to your an existing (non polish) project folder and open the folder nbproject. There should be a file called project.properties, open this with a text editor. Scroll down and there should be two lines stating:
platform.active=Sun_Java_TM__Wireless_Toolkit_2_5_2_for_CLDC
platform.active.description=Sun
Java(TM) Wireless Toolkit 2.5.2 for CLDC
The versions may be different, as you may have a later Wireless Toolkit!
Now open the same file in your J2ME Polish project and replace the platform.active and platform.active.description lines with the ones from the non polish project.
Replace every occurance of “MPowerPlayer” with Sun_Java_TM__Wireless_Toolkit_2_5_2_for_CLDC (or what you had in your old project.)
Replace every occurance of “Emulator Platform” with Sun Java(TM) Wireless Toolkit 2.5.2 for CLDCÂ (or what you had in your old project.)
Close Netbeans and reopen and all should be fixed!