text: Title: `add_marker` not working while plot is not active
  Body: I am having this issue with the `add_marker()` method of `Signal1D` while working on Jupyter notebooks:
  ```python
  import hyperspy.api as hs
  import numpy as np
  
  s = hs.signals.Signal1D(np.arange(500).reshape([5,10,10]))
  
  marker_array = np.tile(np.arange(10), (5,1))
  m = hs.plot.markers.vertical_line(x=marker_array)
  s.add_marker(m)
  ```
  
  The above code works perfectly fine if it is run in a single cell; the signal will be created and plotted with the markers in place. However if I split the code in different cells like below:
  ```python
  # cell no. 1
  import hyperspy.api as hs
  import numpy as np
  
  s = hs.signals.Signal1D(np.arange(500).reshape([5,10,10]))
  
  #cell no. 2
  marker_array = np.tile(np.arange(10), (5,1))
  m = hs.plot.markers.vertical_line(x=marker_array)
  s.add_marker(m)
  ```
  and follow these steps:
  1. Create the signal `s` by running cell no. 1.
  2. Run cell no. 2 to create `marker_array` and marker object `m` and add to `s` using `add_marker()`.
  3. Close the plot that appears.
  4. Try to run cell no. 2 again.
  
  I get this error:
  ```python
  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  <ipython-input-36-7302ff075bd5> in <module>
        1 marker_array = np.tile(np.arange(10), (5,1))
        2 m = hs.plot.markers.vertical_line(x=marker_array)
  ----> 3 s.add_marker(m)
  
  c:\users\geri\documents\dev\hyperspy\hyperspy\signal.py in add_marker(self, marker, plot_on_signal, plot_marker, permanent, plot_signal, render_figure)
     5791                     self.plot()
     5792                 if m._plot_on_signal:
  -> 5793                     self._plot.signal_plot.add_marker(m)
     5794                 else:
     5795                     if self._plot.navigator_plot is None:
  
  AttributeError: 'NoneType' object has no attribute 'add_marker'
  ```
  
  Now if I run `s.plot()` and then run cell no. 2 again this error disappears and the markers are put on the plot, but it will always give me this error if the plot isn't active.
  
  Any ideas?