[Collapse] Warn when collapsedHeight misses units

[popuguytheparrot]

When click on hide after show, the list is not yet collapsed to the desired height

Current Behavior 😯

if click hide to hide height in styles will not change

Expected Behavior 🤔

height remove and sets only min-height in styles

Steps to Reproduce 🕹

https://codesandbox.io/s/keen-thompson-9k8kx

Your Environment 🌎

Tech	Version
Material-UI	latest
React	latest
Browser	chrome latest

[oliviertassinari]

@popuguytheparrot Solve the warnings in the console, and you should be good.

---

However, this surface an opportunity to do better. I would suggest something like this:

diff --git a/packages/material-ui/src/Collapse/Collapse.js b/packages/material-ui/src/Collapse/Collapse.js
index 7295e2045..91300af14 100644
--- a/packages/material-ui/src/Collapse/Collapse.js
+++ b/packages/material-ui/src/Collapse/Collapse.js
@@ -69,6 +69,16 @@ const Collapse = React.forwardRef(function Collapse(props, ref) {

   const handleEnter = (node, isAppearing) => {
     node.style.height = collapsedHeight;
+    if (process.env.NODE_ENV !== 'production') {
+      if (node.style.height !== collapsedHeight) {
+        console.error(
+          [
+            'Material-UI: the `collapsedHeight` prop is invalid.',
+            `You have provided ${collapsedHeight} but it requires units, e.g. '0px'.`,
+          ].join('\n'),
+        );
+      }
+    }

     if (onEnter) {
       onEnter(node, isAppearing);
@@ -138,6 +148,16 @@ const Collapse = React.forwardRef(function Collapse(props, ref) {
     }

     node.style.height = collapsedHeight;
+    if (process.env.NODE_ENV !== 'production') {
+      if (node.style.height !== collapsedHeight) {
+        console.error(
+          [
+            'Material-UI: the `collapsedHeight` prop is invalid.',
+            `You have provided ${collapsedHeight} but it requires units, e.g. '0px'.`,
+          ].join('\n'),
+        );
+      }
+    }

     if (onExiting) {