Sorry about this issue which only manifests itself on older PHP version (before 5.3). We’ve already submitted the fix with a new update, now it’s up to WordPress to approve it and make it live.
Another workaround until then if you’re not into file editing is to delete the theme folder completely (using a file manager or via FTP) and manually install an older theme version: https://downloads.wordpress.org/theme/tempera.1.2.6.zip