I have faced this type of scenario and it will happen if we are in interview screen for long time with (inactive mode) and when we try to "Preview". You can try being at interview screen (some time inactive) and you can reproduce this type of Error.
This issue can sometime occur (in the debugger only) when there are no instances of the entity that would be displayed on that screen. If you create an instance in the debugger before previewing the screen you should not see this error. You don't say what version of OPA you are using but this was fixed in 10.4.1.