While this often repros with a proper URL, easiest way I was able to repro this is:
1) Create a radio station entry with the URL:
   http://78.70.155.96.
2) Click Play. You will get the "search for suitable plugins dialog". Cancel it & the red invalid marker shows up in the radio station list
3) Select some other radio station
4) Re-select the invalid one & click play. Promptly it SEGVs.

GDB reveals the problem to be in rb_shell_player_playing_changed_cb (rb-shell-player.c), line 3769 (as of the v0.13 release).
Changing the following line
		if (rb_source_can_pause (player->priv->source)) {
to
		if (player->priv->source && rb_source_can_pause (player->priv->source)) {

seems to fix this.