Unable to continue a old session

[Nekhelesh Ramananthan]

The restore session provides testers with 3 buttons, "Rerun last", "Continue" and "Restart". The "Continue" button doesn't work and leaves the app stuck with no choice but to restart the app.

Steps to reproduce bug:
1. Open checkbox-touch and select a test plan.
2. Once you see the list of tests, close the app.
3. Reopen app
4. You should see the restore session page.
5. Press "Continue"

At this stage, the app is stuck with the buttons not accepting any input forcing the tester to just restart the app.

[Nicholas Skaggs (nskaggs)]

qml: Getting reference to python object via checkbox_touch.get_qml_logger
qml: invoking get_version_pair on object created withcheckbox_touch.create_app_object, with args: [] ...
INFO:checkbox.touch.qml:get_version_pair on object created withcheckbox_touch.create_app_object, with args: [] returned: {"code":200,"result":{"application_version":"1.2.1","plainbox_version":"0.23.dev"}}
INFO:checkbox.touch.qml:Plainbox version 0.23.dev
INFO:checkbox.touch.qml:Checkbox Touch version 1.2.1
INFO:checkbox.touch.qml:invoking is_session_resumable on object created withcheckbox_touch.create_app_object, with args: [] ...
INFO:checkbox.touch.qml:is_session_resumable on object created withcheckbox_touch.create_app_object, with args: [] returned: {"code":200,"result":{"error_encountered":false,"resumable":true}}
INFO:checkbox.touch.qml:invoking resume_session on object created withcheckbox_touch.create_app_object, with args: [false] ...
INFO:plainbox.secure.providers.v1:Loading content for provider 2013.com.canonical.plainbox:manifest, version 1.0
INFO:plainbox.secure.providers.v1:Loading content for provider 2013.com.canonical.plainbox:exporters, version 1.0
INFO:plainbox.secure.providers.v1:Loading content for provider 2013.com.canonical.plainbox:categories, version 1.0
INFO:plainbox.secure.providers.v1:Loading content for provider 2013.com.canonical.certification:checkbox, version 0.21.3
INFO:plainbox.secure.providers.v1:Loading content for provider 2013.com.canonical.certification:plainbox-resources, version 0.19
INFO:plainbox.secure.providers.v1:Loading content for provider 2014.com.ubuntu.clock:clock-tests, version 1.0
INFO:plainbox.secure.providers.v1:Loading content for provider 2015.com.ubuntu.dekko:dekko-tests, version 1.0
INFO:plainbox.secure.providers.v1:Loading content for provider 2015.com.ubuntu.music:music-tests, version 1.0
INFO:plainbox.secure.providers.v1:Loading content for provider 2015.com.ubuntu.weather:weather-tests, version 1.0
ERROR:checkbox.touch:resume_session(...) raised AttributeError("'NoneType' object has no attribute 'decode'",)
Traceback (most recent call last):
  File "/home/nskaggs/projects/ubuntutouch/merges/lbuilding-on-oauth-community-testing/checkbox-touch/py/checkbox_touch.py", line 96, in wrapper
    result = func(*args, **kwargs)
  File "/home/nskaggs/projects/ubuntutouch/merges/lbuilding-on-oauth-community-testing/checkbox-touch/py/checkbox_touch.py", line 187, in resume_session
    app_blob = json.loads(metadata.app_blob.decode("UTF-8"))
AttributeError: 'NoneType' object has no attribute 'decode'
INFO:checkbox.touch.qml:resume_session on object created withcheckbox_touch.create_app_object, with args: [false] returned: {"code":500,"error":"'NoneType' object has no attribute 'decode'","traceback":"Traceback (most recent call last):\n File \"/home/nskaggs/projects/ubuntutouch/merges/lbuilding-on-oauth-community-testing/checkbox-touch/py/checkbox_touch.py\", line 96, in wrapper\n result = func(*args, **kwargs)\n File \"/home/nskaggs/projects/ubuntutouch/merges/lbuilding-on-oauth-community-testing/checkbox-touch/py/checkbox_touch.py\", line 187, in resume_session\n app_blob = json.loads(metadata.app_blob.decode(...