Try to open some help patches from the help browser that are using Stride, some make the runtime stop when they are opened a second time in a row (for example the Overview 2D and 3D primitive patches). Also the RootScene gets purple sometime, saying its not set to an instance of an object.
It also happens when coping their content to a new patch, saving this one and opening it again.
This problem comes up the first time with preview 0618 - its fine in preview 0600.