gTile not shown with error "grids is undefined"

[leesei]

I encountered this error when I press Super+space to invoke gTile. No pop up is shown. I'm using Mint 15 64-bit.

$ cinnamon --version                
Cinnamon 1.8.8

$ tail -n 10 ~/.xsession-errors    
"'
[WARNING:flash/platform/pepper/pep_module.cpp(63)] SANDBOXED
    JS ERROR: !!!   Exception was: TypeError: grids is undefined
    JS ERROR: !!!     message = '"grids is undefined"'
    JS ERROR: !!!     fileName = '"/home/leesei/.local/share/cinnamon/extensions/gTile@shuairan/extension.js"'
    JS ERROR: !!!     lineNumber = '497'
    JS ERROR: !!!     stack = '"showTiling()@/home/leesei/.local/share/cinnamon/extensions/gTile@shuairan/extension.js:497
toggleTiling([object _private_Meta_Display],[object _private_Meta_Screen],null,[object _private_Meta_KeyBinding])@/home/leesei/.local/share/cinnamon/extensions/gTile@shuairan/extension.js:556
"'
[WARNING:flash/platform/pepper/pep_module.cpp(63)] SANDBOXED