Theme popup not opening when localStorage has an invalid theme id stored

[Pistonight]

Problem

If mdbook-theme in localStorage contains a theme id that's not in the book, an uncaught exception causes theme popup to not function properly

Steps

1. Create a new project with --theme, build and serve the book
2. Switch the theme so mdbook-theme gets set in localStorage (may not be needed?)
3. Edit theme/index.hbs, find <ul id="theme-list" and remove/change the id of the button so that the saved theme is no longer in the list
4. Rebuild the book
5. Observe that clicking on the theme button no longer opens the menu

Possible Solution(s)

The root cause is updateThemeSelected() throws if the selected theme is not in the DOM

I think the cleanest fix is to update get_theme() to return default_theme if the saved theme is invalid

    // cache the valid theme ids
    var themeIds = [];
    themePopup.querySelectorAll('button.theme').forEach(function (el) {
        themeIds.push(el.id);
    });

   // ...

    function get_theme() {
        var theme;
        try { theme = localStorage.getItem('mdbook-theme'); } catch (e) { }
        if (theme === null || theme === undefined || !themeIds.includes(theme)) { // <- add this check
            return default_theme;
        } else {
            return theme;
        }
    }

Verified that the solution works

Notes

No response

Version

mdbook v0.4.40

[Pistonight]

@ehuss pinging according to the contribution guide. Should I open a PR for this fix?

[ehuss]

Seems like a reasonable fix to me, would appreciate a PR!