Couldn't load plugin processing due to an error when calling its classFactory() method 

TypeError: coercing to Unicode: need string or buffer, NoneType found 
Traceback (most recent call last):
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/qgis/utils.py", line 333, in startPlugin
    plugins[packageName] = package.classFactory(iface)
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/__init__.py", line 38, in classFactory
    return ProcessingPlugin(iface)
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/ProcessingPlugin.py", line 58, in __init__
    Processing.initialize()
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/core/Processing.py", line 148, in initialize
    Processing.addProvider(c())
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/core/Processing.py", line 103, in addProvider
    % (provider.getDescription(), traceback.format_exc()))
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/algs/saga/SagaAlgorithmProvider.py", line 123, in getDescription
    version = SagaUtils.getSagaInstalledVersion()
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/algs/saga/SagaUtils.py", line 130, in getSagaInstalledVersion
    commands = [os.path.join(sagaPath(), "saga_cmd -v")]
  File "/Applications/MacPorts/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/algs/saga/SagaUtils.py", line 75, in sagaPath
    if not os.path.isdir(folder):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/genericpath.py", line 49, in isdir
    st = os.stat(s)
TypeError: coercing to Unicode: need string or buffer, NoneType found