Gazebo freezes on Save As with too many cameras

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Hello,

I have installed Gazebo 9 with ROS melodic on Ubuntu 18.04. I keep getting following unexpected behaviour:

1. Create a new world in Gazebo (only ground plane and sun)
2. Add an object to the world (any object e.g. a car)
3. Add cameras to the world (4 cameras needed, from standard library)
4. File -> Save As... or Ctrl+Shift+S
5. The program freezes and the saving window does not show up
6. gzclient process starts using 100% CPU

It is possible to Alt+F4 to return to the world editor. The only thing showing up in the terminal: Gtk-Message: 16:30:04.390: GtkDialog mapped without a transient parent. This is discouraged.

What can be the cause? How to fix this?