Update open methods to return nullable String

[Leon-JavaScript]

This PR fixes the following issue sometimes occurring on Minestom servers:

java.lang.NullPointerException: internalOpen(...) must not be null at
me.devnatan.inventoryframework.ViewFrame.open(ViewFrame.kt:85) at
me.devnatan.inventoryframework.ViewFrame.open(ViewFrame.kt:47) at
me.devnatan.inventoryframework.ViewFrame.open(ViewFrame.kt:33)

This happens because in ViewFrame.java#L160 the internalOpen method can possibly return null, but the open method in ViewFrame.kt#L85 wants to always return a String and not null.

---

Closes #767

[devnatan]

LGTM thx @Leon-JavaScript :))