Setting State Vars that are not defined should raise an error

reflex/state.py

@@ -1277,6 +1277,9 @@ class BaseState(Base, ABC, extra=pydantic.Extra.allow):
             self._mark_dirty()
             return
 
+        if not name in self.vars and not name in self._computed_var_dependencies and not name in self.get_skip_vars():
+            raise AttributeError(f"The state var '{name}' has not been defined in '{type(self).__name__}'. All state vars must be declared before they can be set.")
+
         # Set the attribute.
         super().__setattr__(name, value)
 

tests/units/test_state.py

@@ -3262,3 +3262,26 @@ def test_child_mixin_state() -> None:
 
     assert "computed" in ChildUsesMixinState.inherited_vars
     assert "computed" not in ChildUsesMixinState.computed_vars
+
+
+def test_assignment_to_undeclared_vars():
+    """Test that an attribute error is thrown when undeclared vars are set"""
+    class State(BaseState):
+        val: str
+
+        def handle(self):
+            self.num = 5
+
+    class Substate(State):
+
+        def handle_var(self):
+            self.value = 20
+
+    state = State()
+    sub_state = Substate()
+
+    with pytest.raises(AttributeError):
+        state.handle()
+
+    with pytest.raises(AttributeError):
+        sub_state.handle()