Fix savegame load error on missing mod website

Not a proper fix, but this thing needs a huge refactor anyway. Goodbye,
a bug that existed ever since the first version of mod support!

src/js/states/main_menu.js

@@ -538,7 +538,7 @@ export class MainMenuState extends GameState {
                 <div class="dialogModsMod">
                     <div class="name">${mod.name}</div>
                     <div class="version">${T.mods.version} ${mod.version}</div>
-                    <button class="website styledButton" onclick="window.open('${mod.website.replace(
+                    <button class="website styledButton" onclick="window.open('${mod.website?.replace(
                         /"'/,
                         ""
                     )}')">${T.mods.modWebsite}