Kernel not initialized when extension loads

[filippomc]

Sometimes, randomly, the kernel is not yet initialized when the extension is loading, causing an error when sending the first Python commands

events.js:33 Exception in event handler for notebook_loaded.Notebook TypeError: Cannot read property 'execute' of null
    at load_extension (index.js:26)
    at window._Events.<anonymous> (index.js:40)
    at window._Events.dispatch (jquery.min.js:2)
    at window._Events.y.handle (jquery.min.js:2)
    at Object.trigger (jquery.min.js:2)
    at window._Events.<anonymous> (jquery.min.js:2)
    at Function.each (jquery.min.js:2)
    at w.fn.init.each (jquery.min.js:2)
    at w.fn.init.trigger (jquery.min.js:2)
    at w.fn.init.events.trigger (events.js:31) Arguments ["notebook_loaded.Notebook", callee: (...), Symbol(Symbol.iterator): ƒ]