move the emptyContentEditable hack to componentWillUnmount

ts/components/session/RegistrationTabs.tsx

@@ -549,8 +549,6 @@ export class RegistrationTabs extends React.Component<{}, State> {
             displayName: '',
             signUpMode: SignUpMode.Default,
           });
-          //FIXME ugly hack to empty the content editable div used on enter session ID
-          window.Session.emptyContentEditableDivs();
         }}
         buttonType={buttonType}
         buttonColor={buttonColor}

ts/components/session/SessionIdEditable.tsx

@@ -1,14 +1,16 @@
 import React from 'react';
 
-
 interface Props {
   placeholder: string;
   editable?: boolean;
   onChange?: any;
 }
 
-
 export class SessionIdEditable extends React.PureComponent<Props> {
+  public componentWillUnmount() {
+    //FIXME ugly hack to empty the content editable div used on enter session ID
+    window.Session.emptyContentEditableDivs();
+  }
 
   public render() {
     const { placeholder, editable, onChange } = this.props;