You are not the first person to report this bug, but we have been unable to reproduce/fix it. It seems to happen to a few limited set of people who maybe get their localStorage corrupted somehow.
You should be able to fix it by clearing out the following directory depending on your operating system and reinstalling.
C:\Documents and Settings\<username>\Local Settings\Application Data\Sencha, Inc\Sencha Designer 2.0\
C:\Users\<username>\AppData\Local\Sencha, Inc\Sencha Designer 2.0\
~/.local/share/data/Sencha, Inc./Sencha Designer 2.0/
Mac OS X
~/Library/Application Support/Sencha, Inc./Sencha Designer 2.0/
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