Histogram visualization error when using numeric type.

[kuckjwi0928]

Histogram visualization error when using numeric type.

Query parameter

e.g.
Numeric Columns: Numeric type columns
Group by: Numeric type columns

Expected results

A histogram is drawn.

Actual results

Error occurred. (type error)

Screenshots

How to reproduce the bug

1. Go to Sources menu.
2. Click on Tables.
3. Click on Specific table.
4. Select histogram chart.
5. Numeric Columns and Group by columns setting.
6. Run query
7. See error. ('int' object is not iterable)

Environment

• superset version: master
• python version: Python 3.7.4
• node.js version: v11.5.0
• npm version: 6.9.0

Checklist

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

Additional context

No

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[kuckjwi0928]

This problem will be solved by checking the instance of numeric type.