gistfile1.txt

View creation failed 
Backbone.Model.extend.constructor {widget_manager: WidgetManager, _buffered_state_diff: Object, pending_msgs: 0, msg_throttle: 3, msg_buffer: null…}
 widgetmanager.js:83
Exception in Comm callback 
TypeError {stack: "TypeError: Cannot read property '$el' of null↵    …min.js?v=ccd0edd113b78697e04fb5c1b519a5cd:4:5488)", message: "Cannot read property '$el' of null"}
 TypeError: Cannot read property '$el' of null
    at WidgetManager._handle_display_view (http://127.0.0.1:8888/static/notebook/js/widgetmanager.js:97:58)
    at WidgetManager.display_view (http://127.0.0.1:8888/static/notebook/js/widgetmanager.js:87:26)
    at Backbone.Model.extend._handle_comm_msg (http://127.0.0.1:8888/static/notebook/js/widgets/widget.js:86:41)
    at x.isFunction.i (http://127.0.0.1:8888/static/components/jquery/jquery.min.js?v=ccd0edd113b78697e04fb5c1b519a5cd:4:5488)
    at Comm._maybe_callback (http://127.0.0.1:8888/static/services/kernels/js/comm.js?v=f6acfac4110c05a2d4985876fb49765a:177:17)
    at Comm.handle_msg (http://127.0.0.1:8888/static/services/kernels/js/comm.js?v=f6acfac4110c05a2d4985876fb49765a:185:14)
    at CommManager.comm_msg (http://127.0.0.1:8888/static/services/kernels/js/comm.js?v=f6acfac4110c05a2d4985876fb49765a:116:18)
    at x.isFunction.i (http://127.0.0.1:8888/static/components/jquery/jquery.min.js?v=ccd0edd113b78697e04fb5c1b519a5cd:4:5488)
    at Kernel._handle_iopub_message (http://127.0.0.1:8888/static/services/kernels/js/kernel.js?v=f6186ba2f4410fa59763d501b96e03c4:580:13)
    at WebSocket.x.isFunction.i (http://127.0.0.1:8888/static/components/jquery/jquery.min.js?v=ccd0edd113b78697e04fb5c1b519a5cd:4:5488) 
Object {parent_header: Object, msg_type: "comm_msg", msg_id: "9eef36bd-f2a5-4c63-9930-6bc4182c2a3c", content: Object, header: Object…}
 comm.js?v=f6acfac4110c05a2d4985876fb49765a:179

widget_mapp.js

//----------------------------------------------------------------------------
//  Copyright (C) 2013 The IPython Development Team
//
//  Distributed under the terms of the BSD License.  The full license is in
//  the file COPYING, distributed as part of this software.
//----------------------------------------------------------------------------

//============================================================================
// ButtonWidget
//============================================================================

/**
 * @module IPython
 * @namespace IPython
 **/

define(["notebook/js/widgets/widget"], function(WidgetManager){

    var MappView = IPython.DOMWidgetView.extend({
        render : function(){
            // Called when view is rendered.
            this.setElement($("<button />")
                .addClass('btn'));

            this.update(); // Set defaults.
        },
        
        update : function(){
            // Update the contents of this view
            //
            // Called when the model is changed.  The model may have been 
            // changed by another view or by a state update from the back-end.
            var description = this.model.get('description');
            if (description.length === 0) {
                this.$el.text(' '); // Preserve button height
            } else {
                this.$el.text(description);
            }
            
            if (this.model.get('disabled')) {
                this.$el.attr('disabled','disabled');
            } else {
                this.$el.removeAttr('disabled');
            }

            return MappView.__super__.update.apply(this);
        },

        events: {
            // Dictionary of events and their handlers.
            'click': '_handle_click',
        },
        
        _handle_click: function(){
            // Handles when the button is clicked.
            this.send({event: 'click'});
        },
    });
    WidgetManager.register_widget_view('MappView', MappView);
});

widget_mapp.py

"""MappWidget class.  

Represents a button in the frontend using a widget.  Allows user to listen for
click events on the button and trigger backend code when the clicks are fired.
"""
#-----------------------------------------------------------------------------
# Copyright (c) 2013, the IPython Development Team.
#
# Distributed under the terms of the Modified BSD License.
#
# The full license is in the file COPYING.txt, distributed with this software.
#-----------------------------------------------------------------------------

#-----------------------------------------------------------------------------
# Imports
#-----------------------------------------------------------------------------
from .widget import DOMWidget, CallbackDispatcher
from IPython.utils.traitlets import Unicode, Bool

#-----------------------------------------------------------------------------
# Classes
#-----------------------------------------------------------------------------
class MappWidget(DOMWidget):
    _view_name = Unicode('MappView', sync=True)

    # Keys
    description = Unicode('', help="Description of the button (label).", sync=True)
    disabled = Bool(False, help="Enable or disable user changes.", sync=True)
    
    def __init__(self, **kwargs):
        """Constructor"""
        super(MappWidget, self).__init__(**kwargs)
        self._click_handlers = CallbackDispatcher()
        self.on_msg(self._handle_button_msg)

    def on_click(self, callback, remove=False):
        """Register a callback to execute when the button is clicked.

        The callback will be called with one argument,
        the clicked button widget instance.

        Parameters
        ----------
        remove : bool (optional)
            Set to true to remove the callback from the list of callbacks."""
        self._click_handlers.register_callback(callback, remove=remove)

    def _handle_button_msg(self, _, content):
        """Handle a msg from the front-end.

        Parameters
        ----------
        content: dict
            Content of the msg."""
        if content.get('event', '') == 'click':
            self._click_handlers(self)