Error in plot_xxx() ops 'NoneType' object is not iterable

[hans-permana]

Expected behavior

Plot function to work

Actual behavior

The following exception was returned.

Job: Executing operation "plot_contour"
Message: 'NoneType' object is not iterable
Code: -32003
Method: set_workspace_resource
Exception: TypeError

Traceback (most recent call last):
  File "F:\BC\Cate\cate-core\cate\util\web\jsonrpchandler.py", line 209, in send_service_method_result
    result = future.result()
  File "C:\Miniconda3\envs\cate-env\lib\concurrent\futures\_base.py", line 425, in result
    return self.__get_result()
  File "C:\Miniconda3\envs\cate-env\lib\concurrent\futures\_base.py", line 384, in __get_result
    raise self._exception
  File "C:\Miniconda3\envs\cate-env\lib\concurrent\futures\thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "F:\BC\Cate\cate-core\cate\util\web\jsonrpchandler.py", line 306, in call_service_method
    result = method(*method_params, monitor=monitor)
  File "F:\BC\Cate\cate-core\cate\webapi\websocket.py", line 291, in set_workspace_resource
    monitor=monitor)
  File "F:\BC\Cate\cate-core\cate\core\wsmanag.py", line 317, in set_workspace_resource
    workspace.execute_workflow(res_name=res_name, monitor=monitor)
  File "F:\BC\Cate\cate-core\cate\core\workspace.py", line 644, in execute_workflow
    self.workflow.invoke_steps(steps, context=self._new_context(), monitor=monitor)
  File "F:\BC\Cate\cate-core\cate\core\workflow.py", line 632, in invoke_steps
    step.invoke(context=context, monitor=monitor.child(work=1))
  File "F:\BC\Cate\cate-core\cate\core\workflow.py", line 318, in invoke
    self._invoke_impl(_new_context(context, step=self), monitor=monitor)
  File "F:\BC\Cate\cate-core\cate\core\workflow.py", line 980, in _invoke_impl
    return_value = self._op(monitor=monitor, **input_values)
  File "F:\BC\Cate\cate-core\cate\core\op.py", line 216, in __call__
    return_value = self._wrapped_op(**input_values)
  File "F:\BC\Cate\cate-core\cate\ops\plot.py", line 274, in plot_contour
    var_data = get_var_data(var, indexers)
  File "F:\BC\Cate\cate-core\cate\ops\plot_helpers.py", line 100, in get_var_data
    for dim in remaining_dims:
TypeError: 'NoneType' object is not iterable

Steps to reproduce the problem

1. Get Ozone dataset
2. Run plot_contour operation
3. Select O3_du_tot variable and set time=0 as indexers
4. Click Add Step

Specifications

cate-2.0.0-dev.18