What happens when the Open or Save dialog is cancelled within the pythonaddins module?

Answer

Note:
An 'if' clause can be used to determine the next steps within the Python code after a button is clicked. For example: 

Code:
import arcpy
import pythonaddins

class Open(object):
    """Implementation for Dialog_addin.OpenButton (Button)"""
    def __init__(self):
        self.enabled = True
        self.checked = False
    def onClick(self):
        #Define the map document and data frame
        mxd = arcpy.mapping.MapDocument("CURRENT")
        df = arcpy.mapping.ListDataFrames(mxd,"*")[0]

        #Call the open dialog
        openValue = pythonaddins.OpenDialog("Open Dialog", False, r"C:\temp", "Open")
        #Make sure dialog was not canceled.
        if openValue != None:
            #Create the layer 
            rasterLayer = arcpy.mapping.Layer(openValue)
            #Add the layer to the map document
            arcpy.mapping.AddLayer(df, rasterLayer,"TOP")
            # Refresh map document
            arcpy.RefreshActiveView()
            arcpy.RefreshTOC()
            del mxd, df, openValue
        else:
            pythonaddins.MessageBox("No data will be added to the map document.", "Warning Message", 0)
del mxd, df, openValue