TypeError: grid.columns[0] is undefined

Hello guys!

I am usign vaadin grid. I get:

TypeError: grid.columns[0] is undefined

In Firefox as in the title. In Chrome it is working. The code:

var grid = grid || document.querySelector('vaadin-grid');

HTMLImports.whenReady(function() {

// typo3 fluid.. var json = '{f:format.htmlentitiesDecode(value:testJSON)}';
var testJSON = JSON.parse(json); grid.items = testJSON;

// --> ERROR: undefined in firefox
grid.columns[0].renderer = function(cell) {
    //...
};

});

Why?

Solved it using

window.addEventListener('WebComponentsReady', function(e) {

instead of
 

HTMLImports.whenReady(function() {