Unable to read the 'fields' property of null

I've been working with Extjs for about two weeks now and I've been bombarded by several errors (since I'm new to this framework ) and today checking my console , since I have a problem with a combobox, of an item that is selected until clicking on it twice and not once (as it should).

The error notice reads as follows:

  

Uncaught TypeError: Can not read property 'fields' of null'

Which, checking my code, is in the next block (note that I will trace the area where exactly it tells me that the possible error would be):

classifyFields: function (field, list) {
        var me = this;
        var type = null;
// el siguiente if, en teoría, es el problema.
        if (field.fields.variableLengthField) {
            type = "variableLenghtField";
        } else if (field.fields.journalSaved) {
            type = "journalField";
        }
        console.log(type, field, list);
        list.forEach(function (item) {
            if (item.name == type) {
                me.dataFields.push({
                    "uuidFielMonitorType": item.uuid,
                    "fieldProperties": field.fields
                });
            }
        });
        console.log(me.dataFields);
    }

Can someone guide me with this? I say, to know if this is really the problem that originates me about the bug with the Combobox , it is necessary to thank them for the help!