text: Title: ExplainerDashboard - AttributeError: 'list' object has no attribute 'keys'
  Body: i can't figure out, why i get the following error:  AttributeError: 'list' object has no attribute 'keys'.
  The dashboard works, but the menus: feature importances and SHAP Summary in Feature Dependence menu not.
  
  ```
  explainer = RegressionExplainer(final_best, X_test, y_test, 
                                  descriptions = feature_descriptions,
                                  target='Durchmesser',
                                  units="mm")
  ```
  
  ![1](https://user-images.githubusercontent.com/77811022/115773395-8bcd6f00-a3b0-11eb-9f78-c230eaa5250b.png)
  ![2](https://user-images.githubusercontent.com/77811022/115773396-8cfe9c00-a3b0-11eb-805f-c72b935ee238.png)
  ![3](https://user-images.githubusercontent.com/77811022/115773401-8d973280-a3b0-11eb-90e5-59811abb020d.png)
  ![4](https://user-images.githubusercontent.com/77811022/115773403-8e2fc900-a3b0-11eb-88f6-66920b3e550f.png)
  
  
  
  The same error i get for all features in dataframe:
  
  AttributeError                            Traceback (most recent call last)
  ~\anaconda3\lib\site-packages\explainerdashboard\dashboard_components\overview_components.py in update_importances(
      depth=None,
      permutation_shap='shap',
      pos_label=None
  )
      245         def update_importances(depth, permutation_shap, pos_label):
      246             depth = None if depth is None else int(depth)
  --> 247             plot =  self.explainer.plot_importances(
          plot = undefined
          global self.explainer.plot_importances = undefined
          global kind = undefined
          permutation_shap = 'shap'
          global topx = undefined
          depth = None
          pos_label = None
      248                         kind=permutation_shap, topx=depth, pos_label=pos_label)
      249             return plot
  
  ~\anaconda3\lib\site-packages\explainerdashboard\explainers.py in inner(
      self=<explainerdashboard.explainers.RegressionExplainer object>,
      *args=(),
      **kwargs={'kind': 'shap', 'pos_label': None, 'topx': None}
  )
       46     def inner(self, *args, **kwargs):
       47         if not self.is_classifier:
  ---> 48             return func(self, *args, **kwargs)
          global func = undefined
          self = <explainerdashboard.explainers.RegressionExplainer object at 0x000001EC4798DEB0>
          args = ()
          kwargs = {'kind': 'shap', 'topx': None, 'pos_label': None}
       49         if 'pos_label' in kwargs:
       50             if kwargs['pos_label'] is not None:
  
  ~\anaconda3\lib\site-packages\explainerdashboard\explainers.py in plot_importances(
      self=<explainerdashboard.explainers.RegressionExplainer object>,
      kind='shap',
      topx=None,
      round=3,
      pos_label=None
  )
     1423             units = ""
     1424         if self.descriptions:
  -> 1425             descriptions = self.description_list(importances_df.Feature)
          descriptions = undefined
          self.description_list = <bound method BaseExplainer.description_list of <explainerdashboard.explainers.RegressionExplainer object at 0x000001EC4798DEB0>>
          importances_df.Feature = 0      ESD
  1      NDZ
  2      NDH
  3    WTemp
  4       LF
  5    UTemp
  6    MTemp
  7      KUZ
  Name: Feature, dtype: object
     1426             return plotly_importances_plot(importances_df, descriptions, round=round, units=units, title=title)
     1427         else:
  
  ~\anaconda3\lib\site-packages\explainerdashboard\explainers.py in description_list(
      self=<explainerdashboard.explainers.RegressionExplainer object>,
      cols=0      ESD
  1      NDZ
  2      NDH
  3    WTemp
  4   ...    MTemp
  7      KUZ
  Name: Feature, dtype: object
  )
      724             list of descriptions
      725         """
  --> 726         return [self.description(col) for col in cols]
          self.description = <bound method BaseExplainer.description of <explainerdashboard.explainers.RegressionExplainer object at 0x000001EC4798DEB0>>
          global col = undefined
          cols = 0      ESD
  1      NDZ
  2      NDH
  3    WTemp
  4       LF
  5    UTemp
  6    MTemp
  7      KUZ
  Name: Feature, dtype: object
      727 
      728     def ordered_cats(self, col, topx=None, sort='alphabet', pos_label=None):
  
  ~\anaconda3\lib\site-packages\explainerdashboard\explainers.py in <listcomp>(.0=<map object>)
      724             list of descriptions
      725         """
  --> 726         return [self.description(col) for col in cols]
          global self.description = undefined
          col = 'ESD'
          global cols = undefined
      727 
      728     def ordered_cats(self, col, topx=None, sort='alphabet', pos_label=None):
  
  ~\anaconda3\lib\site-packages\explainerdashboard\explainers.py in description(
      self=<explainerdashboard.explainers.RegressionExplainer object>,
      col='ESD'
  )
      711             str, description
      712         """
  --> 713         if col in self.descriptions.keys():
          col = 'ESD'
          self.descriptions.keys = undefined
      714             return self.descriptions[col]
      715         return ""
  
  AttributeError: 'list' object has no attribute 'keys'