Description of problem:
After registering, subscribe button in Subscription Assistant can still be clicked when no subscription is selected.

Version-Release number of selected component (if applicable):
subscription-manager-0.96.11-1.el6.x86_64
python-rhsm-0.96.12-1.el6.noarch
candlepin (stage env)

How reproducible:
Always

Steps to Reproduce:
1.register to stage using stage_test_2/redhat
2.# subscription-manager-gui
3.click "Update Certificates" and the Subscription Assistant dialog pops up.
4.check product "Red Hat Enterprise Linux 6 Server" and select its available subscription, then click subscribe button.
5.click subscribe button again.

Actual results:
Subscribe button can be clicked even if no subscription is selected.
The terminal prints error message as follows.
Traceback (most recent call last):
  File "/usr/share/rhsm/subscription_manager/gui/subscription_assistant.py", line 554, in subscribe_button_clicked
    pool_id = model.get_value(tree_iter, self.subscriptions_store['pool_id'])
TypeError: iter must be a GtkTreeIter

Expected results:
Subscribe button should be disabled.

Additional info:
N/A