Sencha Modern 6.6, also 6.0 and Touch 2.3.

If you set src to an empty string or null, an error is thrown (which apparently no ever catches) and the background-image (or img src) is set to the URL of the application's original URL.

The following is an override of updateSrc which corrects the problem. There may be better ways to correct it.
Code:
/*
 * Ext.Img override so that empty src does not set src/url to original app URL.
 */
Ext.define('Ext.override.Img', {
    override: 'Ext.Img',
    updateSrc: function (newSrc) {
        var me = this,
            dom;

        if (me.getMode() === 'background') {
            dom = this.imageObject || new Image();
        } else {
            dom = me.imageElement.dom;
        }

        this.imageObject = dom;

        if (newSrc && Ext.isString(newSrc)) {
            dom.setAttribute('src', newSrc);
            dom.addEventListener('load', me.onLoad, false);
            dom.addEventListener('error', me.onError, false);
        } else {
            if (me.getMode() === 'background') {
                this.element.dom.style.backgroundImage = '';
            } else {
                dom.removeAttribute('src');
            }
        }
    },
});