Steps to reproduce:
Try to run the code below. On third track you will get warning or crash.

import gobject
gobject.threads_init()

import gst

class Player():
    def __init__(self):
        self.player = gst.Pipeline("player")
        source = gst.element_factory_make("filesrc", "filesrc")
        decoder = gst.element_factory_make("decodebin2", "decodebin2")
        conv = gst.element_factory_make("audioconvert", "converter")
        volume = gst.element_factory_make("volume", "volume")
        sink = gst.element_factory_make("autoaudiosink", "alsa-output")

        self.player.add(source, decoder, conv, volume, sink)
        gst.element_link_many(source, decoder)
        gst.element_link_many(conv, volume, sink)

        def new_decode_pad(dbin, pad, islast):
            pad.link(conv.get_pad("sink"))
        decoder.connect("new-decoded-pad", new_decode_pad)

    def play_track(self, track):
        self.player.set_state(gst.STATE_NULL)
        self.player.get_by_name("filesrc").set_property("location", track)
        self.player.set_state(gst.STATE_PLAYING)


from time import sleep
p = Player()
p.play_track('track1.mp3')
sleep(3)
p.play_track('track2.mp3')
sleep(3)
p.play_track('track3.mp3')

sleep(10)

Stack trace:
Warning: invalid uninstantiatable type `(null)' in cast to `GObject'
  self.player.set_state(gst.STATE_PLAYING)
Warning: g_object_notify: assertion `G_IS_OBJECT (object)' failed
  self.player.set_state(gst.STATE_PLAYING)

Other information:
This happens only if using decodebin2.