Pressing backspace when entering text wipes out the canvas

The problem you are facing is most probably already fixed. I remember this problem from years ago. Can you please change the primary color like e.g. to red and do the same steps as above? If this is the same problem, then canvas will be filled with red color. In your case most probably your primary color was black when test was performed.

Please also provide info what is your operating system and version.

If you are on supported Ubuntu distro, then you can upgrade Pinta to 1.6 using a PPA. Do the following in the terminal:
sudo add-apt-repository ppa:pinta-maintainers/pinta-stable
sudo apt-get update
sudo apt-get install pinta

Details about installing are on Pinta Documentation web page: http://pinta-project.com/howto/installing-pinta