A little modification to solve "Segmentation fault"

[oxidationreduction]

Hi. In where2act/code/env.py and ditto/data_generation/env.py, this line of code in Env.close() may cause 'Segmentation fault'.

    def close(self):
        if self.show_gui:
            self.viewer.set_scene(None) # <- this line
        self.scene = None

Because it will pass a NoneType to a viewer, and the viewer will call self.window.set_scene(scene) to set the scene of the window to a null object, causing illegal memory access. Here's sapien's C++ source code.

void SVulkan2Window::setScene(SVulkan2Scene *scene) {
  mScene = scene;
  mSVulkanRenderer->setScene(mScene->getScene()); // mScene is null, crashed
}

So I suggest change Env.close() to

    def close(self):
        if self.show_gui:
            self.viewer.close()
        self.scene = None

to solve the problem.