e.map is not a function explore view

I encountered #2687 while casting from a table visualization with specified columns to a bar chart time series. Refreshing the page gets rid of the issue

Expected results

Moving between viz modes does not crash the slice explorer

Actual results

I reproduced #2687 by attempting to switch from a table viz to a time-series bar chart viz after creating the chart via the explore button in sql lab, and debugged on the exception to see:

How to reproduce the bug

while debugging on all exceptions

1. create a table viz with specified columns and no metrics
2. attempt to switch the viz type to line or time series bar chart
3. Error should be thrown. You should see that e in the console is the above structure

Environment

(please complete the following information):

• superset version: I’m on a slightly modified fork of 0.34.0. Others in the comments of #2687 corroborate. Would be surprised if it had been fixed

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