TypeError: Cannot read property 'getElementsByClassName' of null

[Mr.Sponti]

Hi,
I’m at my wit’s end!
Based on module “MMM-FHEM by BenRoe” I’ve created a module to receive event changes from the home automation pimatic. The module works fine. Then I started to write a second modul and can’t get rid of the error:
Cannot read property ‘getElementsByClassName’ of nullt

To isolate the problem I reduced the code to the example from the module development page.

Module.register("HelloWorld",{
    // Default module config.
    defaults: {
        text: "Hello World!"
    },
    
    start: function() {
        var self = this;
        setInterval(function() {
            self.updateDom(); // no speed defined, so it updates instantly.
        }, 1000); //perform every 1000 milliseconds.
    },
    // Override dom generator.
    getDom: function() {
        var wrapper = document.createElement("div");
        wrapper.className = 'MyHelloWorld';
        wrapper.innerHTML = this.config.text;
        return wrapper;
    }
});

Following the console log:

What’ s wrong? I need help from an expert!

[strawberry 3.141]

@Mr.Sponti that happens because you are trying to update the DOM before its even created

[MichMich]

Try addicting this method to your module:

notificationReceived: function(notification, payload) {
    if (notification === 'DOM_OBJECTS_CREATED') {
        // you can now schedule the dom update method.
    }
}

[Mr.Sponti]

@MichMich: Thank’s a lot, I’ve fixed the issue. Result can be reviewed here