Python add-in won't buffer from output layer

Question

I have a simple add-in script for Arcmap 10.1 to create a toolbar, select a layer from a drop down list of the current layers in the project, buffer that layer four-miles, then use the resulting four-mile buffer output as an input for a six-mile outside buffer. I have a work flow where I repeatedly need to create a four-mile buffer with a six-mile outside buffer surrounding this. Everything functions up to and including the four-mile buffer layer creation, but I get no six-mile outside buffer being created. I have been working with ESRI support but having trouble resolving this. Any ideas?

import arcpy
import pythonaddins
import os

class ComboBoxClass1(object):
    """Implementation for DataRequestBuffer_addin.combobox (ComboBox)"""
    def __init__(self): 
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWWWWWWWWWWWWWWWW'
        self.width = 'WWWWWWWWWWWWWWWWWWWW'
    def onSelChange(self, selection):
        arcpy.env.overwriteOutput = True
        # When a new layer is selected, create a 4 mile buffer around that layer then create a 6 mile buffer surrounding the outside of the 4 mile buffer.
        layer = arcpy.mapping.ListLayers(self.mxd, selection)[0]
        outPath = os.path.dirname(layer.dataSource)
        print outPath

        # Buffer 4 miles
        print 'Buffering %s with 4 mile buffer' % layer
        arcpy.Buffer_analysis(layer, outPath + os.sep + str(layer) + "_4milebuff", "4 Miles", "FULL", "ROUND", "ALL", "")
        buff6 = outPath + os.sep + str(layer) + "_4milebuff"
        # Buffer 6 miles surrounding outside of 4 mile buffer
        print 'Buffering %s with 6 mile outside buffer' % buff6
        arcpy.Buffer_analysis(buff6, outPath + os.sep + str(layer) + "_6milebuffoutside", "6 Miles", "OUTSIDE_ONLY", "ROUND", "ALL", "")
        arcpy.RefreshActiveView()


    def onEditChange(self, text):
        pass
    def onFocus(self, focused):
      # When the combo box has focus, update the combo box with the list of layer names.
        if focused:
                self.mxd = arcpy.mapping.MapDocument('current')
                layers = arcpy.mapping.ListLayers(self.mxd)
                self.items = []
                for layer in layers:
                        self.items.append(layer.name)

    def onEnter(self):
        pass
    def refresh(self):
        pass

Answer 1

The following worked for a file geodatabase but not for a shapefile, now working to get it to work with a shapefile. Suspect a naming convention. The geoprocessing is working with a shapefile when I put the code in the python window and changed the output path to a set location for example: r"V:\ActiveProjects\4milebuff". So no exception in the Python window. Have not tried GetMessages yet. Got bogged down with other work. Will post results.

import arcpy
import pythonaddins
import os

class ComboBoxClass1(object):
    """Implementation for DataRequestBuffer_addin.combobox (ComboBox)"""
    def __init__(self): 
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWWWWWWWWWWWWWWWW'
        self.width = 'WWWWWWWWWWWWWWWWWWWW'
    def onSelChange(self, selection):
        arcpy.env.overwriteOutput = True
    # When a new layer is selected, create a 4 mile buffer around that layer then create a 6 mile buffer surrounding the outside of the 4 mile buffer.
        layer = arcpy.mapping.ListLayers(self.mxd, selection)[0]
        outPath = os.path.dirname(layer.dataSource)
        print outPath

    # Buffer 4 miles
    print 'Buffering %s with 4 mile buffer' % layer
    arcpy.Buffer_analysis(layer, outPath + os.sep + str(layer) + "_4milebuff", "4 Miles", "FULL", "ROUND", "ALL", "")
    buff6 = outPath + os.sep + str(layer) + "_4milebuff"
    # Buffer 6 miles surrounding outside of 4 mile buffer
    print 'Buffering %s with 6 mile outside buffer' % buff6
    arcpy.Buffer_analysis(buff6, outPath + os.sep + "_6milebuffoutside", "6 Miles", "OUTSIDE_ONLY", "ROUND", "ALL", "")
arcpy.RefreshActiveView()

def onEditChange(self, text):
    pass
def onFocus(self, focused):
  # When the combo box has focus, update the combo box with the list of layer names.
if focused:
    self.mxd = arcpy.mapping.MapDocument('current')
    layers = arcpy.mapping.ListLayers(self.mxd)
    self.items = []
    for layer in layers:
        self.items.append(layer.name)

def onEnter(self):
    pass
def refresh(self):
    pass