sqllab bug: 'unicode' has no attribute 'get_table_names'

[tejaser]

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

• I have checked the superset logs for python stacktraces and included it here as text if 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

Superset version

0.17.5 running on python 2.7 in ubuntu 16

Expected results

Table should get listed in the dropdown

Actual results

spinner keeps on rotating waiting for data to come

Steps to reproduce

step 1:- Go to sqllab
step 2:- select the database and schema
step 3:- wait for the spinner to start rotating

Error log

File "/usr/local/lib/python2.7/dist-packages/superset/views/base.py", line 48, in wraps
return f(self, *args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/flask_appbuilder/security/decorators.py", line 52, in wraps
return f(self, *args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/superset/views/core.py", line 1212, in tables
database, database.all_table_names(schema), schema)
File "/usr/local/lib/python2.7/dist-packages/superset/models/core.py", line 631, in all_table_names
self.db_engine_spec.get_table_names(schema, self.inspector))
File "/usr/local/lib/python2.7/dist-packages/superset/db_engine_specs.py", line 307, in get_table_names
return sorted(inspector.get_table_names())
AttributeError: 'unicode' object has no attribute 'get_table_names'

[xrmx]

Is this a sqlite database? Looks like a dup of #2664