Hi this is caused by a bug in Unity which is happeningly alarmingly more often
You can fix it by:
1. Click on TK2DROOT/tk2dUI_demo/Assets/Spritecollection/UIDemoSpriteCollection
2. Click Open Editor...
3. You will notice that under the fonts section, you may see one or more “Empty” entries. Click on it
4. Note the BMFont field is set to None, this is the link that has been lost. Click #1 as shown in the attached image to highlight and expand the folder in the project window, and drag #2 as shown in the image below into the slot.
5. Click Commit to commit the sprite collection, and the fonts should all fix themselves in the scene.