Create Layout that does not contain any Window object

I want to initialize my layout with an empty HSplit. Children are added and removed later while the application is running. The first child is not available when I create the Layout.

>>> from prompt_toolkit.layout import Layout
>>> from prompt_toolkit.layout.containers import HSplit
>>> Layout(HSplit(children=[]))
Traceback (most recent call last):
  File "/home/ich/code/upsies/venv/lib/python3.8/site-packages/prompt_toolkit/layout/layout.py", line 61, in __init__
    self._stack.append(next(self.find_all_windows()))
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ich/code/upsies/venv/lib/python3.8/site-packages/prompt_toolkit/layout/layout.py", line 63, in __init__
    raise InvalidLayoutError(
prompt_toolkit.layout.layout.InvalidLayoutError: Invalid layout. The layout does not contain any Window object.

I tried replacing the exception with a pass in https://github.com/prompt-toolkit/python-prompt-toolkit/blob/10f5cca40f40f5dbda4c521ba72270b23f72628c/prompt_toolkit/layout/layout.py#L63 and the tests were passing, but then the current_control property broke. It looks like non-emptiness is built into the guts of the Layout class.

I could add an empty window, but that adds an empty line. I could remove that window when I've added the first real widget, but that's not very elegant or obvious why that's necessary.

Read prompt_toolkit/layout/layout.py, especially the constructor logic that finds a Window and the current_control property. Reproduce the empty HSplit case, inspect the existing layout tests, and define tests showing that children can be added and removed later without requiring a dummy Window or breaking current_control.