If the preview in NiceForm suddenly stops being updated (blank white preview without any data is shown), you should check how many files are there in the Temp folder on your computer. If there are 65535 files in the Temp folder, new files cannot be created. You should remove temporary files that you do not need anymore, and NiceForm will again be able to update previews.
The easiest way to check how many files are in the Temp folder is to type %temp% in the address bar of the Windows Explorer.
Note: This problem only happens on computers that use FAT32 file system. FAT32 has a limit of 65535 files per folder / directory.