[material-ui][Popover] Allow null in anchorEl function return type

packages/mui-material/src/Popover/Popover.d.ts

@@ -108,9 +108,8 @@ export interface PopoverProps
   anchorEl?:
     | null
     | Element
-    | (() => Element)
     | PopoverVirtualElement
-    | (() => PopoverVirtualElement);
+    | (() => Element | PopoverVirtualElement | null);
   /**
    * This is the point on the anchor where the popover's
    * `anchorEl` will attach to. This is not used when the

packages/mui-material/src/Popover/Popover.js

@@ -349,7 +349,7 @@ const Popover = React.forwardRef(function Popover(inProps, ref) {
       setPositioningStyles();
     });
 
-    const containerWindow = ownerWindow(anchorEl);
+    const containerWindow = ownerWindow(resolveAnchorEl(anchorEl));
     containerWindow.addEventListener('resize', handleResize);
     return () => {
       handleResize.clear();

packages/mui-material/src/Popover/Popover.spec.tsx

@@ -71,3 +71,9 @@ function Custom2(props: PopoverProps) {
     </Popover>
   );
 }
+
+function TestAnchorElementFunctionReturnType() {
+  const buttonRef = React.useRef<HTMLButtonElement>(null);
+
+  return <Popover open anchorEl={() => buttonRef.current} />;
+}