Fix Reference & Manager compatibility with React 18's StrictMode (#459)

Author: PetrusAsikainen

src/Manager.js

@@ -15,6 +15,7 @@ export function Manager({ children }: ManagerProps): React.Node {
 
   const hasUnmounted = React.useRef(false);
   React.useEffect(() => {
+    hasUnmounted.current = false;
     return () => {
       hasUnmounted.current = true;
     };

src/Reference.js

@@ -22,10 +22,6 @@ export function Reference({ children, innerRef }: ReferenceProps): React.Node {
     [innerRef, setReferenceNode]
   );
 
-  // ran on unmount
-  // eslint-disable-next-line react-hooks/exhaustive-deps
-  React.useEffect(() => () => setRef(innerRef, null), []);
-
   React.useEffect(() => {
     warning(
       Boolean(setReferenceNode),