Kindle Previewer is a Java application, which seems to require JDK 1.6 and does not run on JDK 1.7.
To fix the environment, open
/Applications/Kindle Previewer.app/Contents/MacOS/Launcher and add JAVA_HOME pointing to your JDK 1.6 installation right after #!/bin/sh, e.g.
Web Apps, SPA, PWA with vanilla Java Script (ES 6+), CSS 3 and WebStandards only. As simple as possible, but not simpler. See you at: (Progressive) Web apps, Single Page Apps and WebStandards airhacks workshops at MUC airport, Winter Edition
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