For some reason my screen have a white screen behind of the UI.
Another image :
These Null exceptions can be the cause?
The code seems ok
A null exception error can always be the cause. It screws up a lot of things under the hood, even completely unrelated things. It’s like giving Unity temporary brain damage.
Looks like you have a lot of errors going on there, including the Dialogue.cs code not being complete (OnValidate, OnBeforeSerialize and OnAfterSerialize are all three throwing NotImplementedExceptions, meaning that the line throw NotImplementedException has not been replaced in those methods.
The highlighted message seems to indicate that the AIConversant does not have a valid Dialogue attached. That may be misleading, however, as it may also mean that the nodeLookup was never properly initialized (see OnValidate → NotImplementedException).