napari: Unable to open another viewer after closing another one in the same program

🐛 Bug

Napari can’t open multiple viewers inside a for loop, note that they are not being executed at the same time. This used to work in 0.4.6, it might be related to #2977.

I’m not sure where to start to fix this.

To Reproduce

Steps to reproduce the behavior:

1. Execute the following code.

import napari

for i in range(10):
    viewer = napari.Viewer()
    napari.run()

1. Close the first viewer.
2. Subsequent viewers do not work as expected, they open all at once.

Expected behavior

To open the next viewer as usual after the previous one was closed.

Environment

napari: 0.4.10
Platform: Linux-5.8.0-59-generic-x86_64-with-glibc2.17
System: Ubuntu 20.04.2 LTS
Python: 3.8.10 (default, Jun 4 2021, 15:09:15) [GCC 7.5.0]
Qt: 5.15.2
PyQt5: 5.15.4
NumPy: 1.21.0
SciPy: 1.7.0
Dask: 2021.05.1
VisPy: 0.7.0

OpenGL:
- GL version: 4.6.0 NVIDIA 460.73.01
- MAX_TEXTURE_SIZE: 32768

Screens:
- screen 1: resolution 3440x1440, scale 1.0

Plugins:
- console: 0.0.3
- napari-dexp: 0.0.1.dev1
- napari-ibacktracker: 0.1.dev20+gb5119c1
- napari-interactive-segm: 0.1.dev3+g23d71b8.d20210712
- napari-simple-crop: 0.1.dev3+g1852e97
- scikit-image: 0.4.10
- svg: 0.1.5