text: Title: Bug with 0.18.0: Matplotlib clabels become NoneType object for figs with projection=ccrs.PlateCarree()
  Body: Calling to ax.clabel plots contour labels as desired, yet the clabel object itself is somehow 'None' for matplotlib figures initilaized with projection=ccrs.PlateCarree(). This becomes an issue because I then wipe my contour and contour label objects clean after each iteration of a long loop, as in [this answer](https://stackoverflow.com/questions/47049296/matplotlib-how-to-remove-contours-clabel), but a TypeError is thrown because of course it can't iterate over a NoneType object. By rolling back to cartopy 0.17.0 and seeing clabel producing an object as intended, I can confirm that this is a bug that appears with version 0.18.0.
  
  Below is an example script that illustrates that contour labels become NoneType objects when projection=ccrs.PlateCarree() is invoked with cartopy 0.18.0. I'm running with Anaconda on a Win64 PC.
  
  #### Code to reproduce
  
  ```
  import matplotlib as mpl
  mpl.use('Agg')
  import matplotlib.pyplot as plt
  import numpy as np
  from time import time
  from datetime import datetime, timedelta
  from siphon.catalog import TDSCatalog
  import cartopy.crs as ccrs
  
  # Recreate the gridded data from the matplotlib contour example
  delta = 0.025
  x = np.arange(-3.0, 3.0, delta)
  y = np.arange(-2.0, 2.0, delta)
  X, Y = np.meshgrid(x, y)
  Z1 = np.exp(-X**2 - Y**2)
  Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
  Z = (Z1 - Z2) * 2
  
  # Contour it and label it to show that labels work and can be removed as desired
  fig, ax = plt.subplots()
  cN = ax.contour(X, Y, Z)
  lbl = ax.clabel(cN)
  #plt.show()
  print("\n\nContour label for a basic 2D plot is: ")
  print(lbl)
  
  # Now remove those labels
  # Will work as intended for lbl
  for label in lbl:
    label.remove()
  
  # Now try a dataset that needs to be geographically referenced
  # Use siphon to get a weather model dataset
  # This dataset link will expire on approximately March 9, 2021
  model_url = "https://www.ncei.noaa.gov/thredds/catalog/model-rap130/202009/20200909/catalog.xml?dataset=rap130/202009/20200909/rap_130_20200909_1800_000.grb2"
  vtime = datetime.strptime('2020090918','%Y%m%d%H')
  
  # Get the data
  model = TDSCatalog(model_url)
  ds = model.datasets[0]
  ncss = ds.subset()
  query = ncss.query()
  query.accept('netcdf')
  query.time(vtime) # Set to the analysis hour only
  query.add_lonlat()
  query.variables('Geopotential_height_isobaric')
  data = ncss.get_data(query)
  
  # Get the lats and lons and a data field from the file
  lats = data.variables['lat'][:,:]
  lons = data.variables['lon'][:,:]
  hght = data.variables['Geopotential_height_isobaric'][0,24,:,:] # 700 hPa is 24th element
  
  # Contour that weather data grid
  # This requires cartopy, which seems to be the problem
  # Redefine the figure, because this time we need to georeference it
  fig = plt.figure(5, figsize=(1600/96,1600/96))
  ax = fig.add_subplot(111, projection=ccrs.PlateCarree())
  cN2 = ax.contour(lons, lats, hght)
  lbl2 = ax.clabel(cN2)
  #plt.show()
  print("\n\nContour label for weather data plot is: ")
  print(lbl2)
  
  # Removing labels will not work for lbl2 because it can't iterate over a NoneType object
  # if using cartopy 0.18.0
  for label in lbl2:
    label.remove()
  ```
  
  #### Traceback 
  
  ```
  Traceback (most recent call last):
    File "clabel_bug.py", line 67, in <module>
      for label in lbl2:
  TypeError: 'NoneType' object is not iterable
  
  
  ```