Fix crash unmounting an empty Portal

packages/react-dom/src/__tests__/ReactDOMFiber-test.js

@@ -461,6 +461,40 @@ describe('ReactDOMFiber', () => {
     expect(container.innerHTML).toBe('<div></div>');
   });
 
+  it('should unmount empty portal component wherever it appears', () => {
+    const portalContainer = document.createElement('div');
+
+    class Wrapper extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = {
+          show: true,
+        };
+      }
+      render() {
+        return (
+          <div>
+            {this.state.show && (
+              <React.Fragment>
+                {ReactDOM.createPortal(null, portalContainer)}
+                <div>child</div>
+              </React.Fragment>
+            )}
+            <div>parent</div>
+          </div>
+        );
+      }
+    }
+
+    const instance = ReactDOM.render(<Wrapper />, container);
+    expect(container.innerHTML).toBe(
+      '<div><div>child</div><div>parent</div></div>',
+    );
+    instance.setState({show: false});
+    expect(instance.state.show).toBe(false);
+    expect(container.innerHTML).toBe('<div><div>parent</div></div>');
+  });
+
   it('should keep track of namespace across portals (simple)', () => {
     assertNamespacesMatch(
       <svg {...expectSVG}>

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -1038,12 +1038,12 @@ function unmountHostComponents(current): void {
       }
       // Don't visit children because we already visited them.
     } else if (node.tag === HostPortal) {
-      // When we go into a portal, it becomes the parent to remove from.
-      // We will reassign it back when we pop the portal on the way up.
-      currentParent = node.stateNode.containerInfo;
-      currentParentIsContainer = true;
-      // Visit children because portals might contain host components.
       if (node.child !== null) {
+        // When we go into a portal, it becomes the parent to remove from.
+        // We will reassign it back when we pop the portal on the way up.
+        currentParent = node.stateNode.containerInfo;
+        currentParentIsContainer = true;
+        // Visit children because portals might contain host components.
         node.child.return = node;
         node = node.child;
         continue;