When a visualizer falls out of scope, ALL visualizer windows close

**Describe the bug**
On Windows, when a visualizer falls out of scope, it closes ALL visualizer windows

**To Reproduce**
Steps to reproduce the behavior:

```
import open3d as o3d

def get_vis():
vis = o3d.visualization.Visualizer()
vis.create_window(width=256,height=256,visible=True)
return vis

v = get_vis() #Opens a window
v = get_vis() #Briefly two windows open, then the first instance of v falls out of scope and its window AND the new window close
```

**Expected behavior**
I expect there to be one open visualizer window. However, there are 0.