Description of problem:
Subscribe button remains active when there is no subscription selected. 

Version-Release number of selected component (if applicable):
subscription-manager-firstboot-0.96.8-1.git.22.6521906.el6.x86_64
subscription-manager-debuginfo-0.96.8-1.git.22.6521906.el6.x86_64
subscription-manager-gnome-0.96.8-1.git.22.6521906.el6.x86_64
subscription-manager-0.96.8-1.git.22.6521906.el6.x86_64

How reproducible:
always

Steps to Reproduce:
There are two ways this can be reproduced. They are slightly different and both should be addressed.

First:
1) Open Subscription Assistant
2) Click Check all - Ensure there are multiple subscriptions to choose from
3) Select a subscription
4) CTRL-click to unselect it.
5) Subscribe button remains active.

Second:
1) Open Subscription Assistant
2) Click Check all - Ensure there are multiple subscriptions to choose from
3) Select a subscription and click subscribe.
4) Notice there are no subscriptions selected, but the 'Subscribe' button remains active.
  
Actual results:
In both cases, the 'Subscribe' button remains active. Clicking it produces the following error in log file.

Traceback (most recent call last):
  File "/usr/share/rhsm/subscription_manager/gui/subscription_assistant.py", line 553, 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 not be active unless there is a subscription selected.

Additional info: