Like the help text of the “Start Dialogue Tree” action reads (shown in the image as well) 🙂 :
“Please Drag & Drop the DialogueTree Component rather than the GameObject for assignment” 🙂
So, please don’t try to assign the DialogueTree reference by dragging and dropping the Dialogue Tree gameobject, but rather drag & drop the DialogueTree component itself after you have selected the Dialogue Tree gameobject.