Cannot create pie chart when there is no date column in the table

[maver1ck]

Make sure these boxes are checked before submitting your issue - thank you!

• [yes] I have checked the superset logs for python stacktraces and included it here as text if any
• [yes] I have reproduced the issue with at least the latest released version of superset
• [yes] I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

0.13.2

Actual results

2016-11-25 14:23:22,778:ERROR:root:local variable 'dttm_col' referenced before assignment
Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/superset/views.py", line 1277, in explore_json
    payload = viz_obj.get_json()
  File "/usr/local/lib/python3.4/dist-packages/superset/viz.py", line 324, in get_json
    'data': self.get_data(),
  File "/usr/local/lib/python3.4/dist-packages/superset/viz.py", line 1234, in get_data
    df = self.get_df()
  File "/usr/local/lib/python3.4/dist-packages/superset/viz.py", line 1226, in get_df
    df = super(DistributionPieViz, self).get_df(query_obj)
  File "/usr/local/lib/python3.4/dist-packages/superset/viz.py", line 169, in get_df
    self.results = self.datasource.query(**query_obj)
  File "/usr/local/lib/python3.4/dist-packages/superset/models.py", line 1103, in query
    inner_time_filter = dttm_col.get_time_filter(
UnboundLocalError: local variable 'dttm_col' referenced before assignment

Steps to reproduce

Create pie chart for table without date column.

Hint: you can do this with sunburst diagram.

[spspaul]

For the record, I got the same exception when I updated from:

%> pip show caravel
---
Metadata-Version: 1.1
Name: caravel
Version: 0.11.0

to

%> pip show superset 2>&1 | head -2
Name: superset
Version: 0.13.2

Once I provided a value for Time Column in the Slice definition
using the Distribution - NVD3 - Pie Chart widget, it started working again.