fix: make sure wrapper is found before accessing the classList (#225)

When used with https://github.com/pmmmwh/react-refresh-webpack-plugin this error shows up during incremental builds.
```Cannot read property 'classList' of null```

Author: serkanyersen

lib/client/overlays/progress-minimal.js

@@ -95,7 +95,9 @@ const init = (options, socket) => {
     const percent = Math.floor(data.percent * 100);
     const wrapper = document.querySelector(`#${ns}`);
 
-    wrapper.classList.remove(`${ns}-hidden`, `${ns}-disappear`);
+    if (wrapper) {
+      wrapper.classList.remove(`${ns}-hidden`, `${ns}-disappear`);
+    }
 
     if (data.percent === 1) {
       if (document.hidden) {