Fix unrecoverable crash when the screen URL param is not a valid screen

packages/builder/src/pages/[application]/frontend/[page]/[screen]/_layout.svelte

@@ -1,6 +1,6 @@
 <script>
   import { onMount } from "svelte"
-  import { params, leftover } from "@sveltech/routify"
+  import { params, leftover, goto } from "@sveltech/routify"
   import { store } from "builderStore"
 
   // Get any leftover params not caught by Routifys params store.
@@ -8,17 +8,30 @@
 
   // It's a screen, set it to that screen
   if ($params.screen !== "page-layout") {
-    store.setCurrentScreen(decodeURI($params.screen))
+    const currentScreenName = decodeURI($params.screen)
+    const validScreen =
+      $store.screens.findIndex(
+        screen => screen.props._instanceName === currentScreenName
+      ) !== -1
 
-    // There are leftover stuff, like IDs, so navigate the components and find the ID and select it.
-    if ($leftover) {
-      // Get the correct screen children.
-      const screenChildren = $store.pages[$params.page]._screens.find(
-        screen =>
-          screen.props._instanceName === $params.screen ||
-          screen.props._instanceName === decodeURIComponent($params.screen)
-      ).props._children
-      findComponent(componentIds, screenChildren)
+    if (!validScreen) {
+      // Go to main layout if URL set to invalid screen
+      store.setCurrentPage("main")
+      $goto("../../main")
+    } else {
+      // Otherwise proceed to set screen
+      store.setCurrentScreen(currentScreenName)
+
+      // There are leftover stuff, like IDs, so navigate the components and find the ID and select it.
+      if ($leftover) {
+        // Get the correct screen children.
+        const screenChildren = $store.pages[$params.page]._screens.find(
+          screen =>
+            screen.props._instanceName === $params.screen ||
+            screen.props._instanceName === decodeURIComponent($params.screen)
+        ).props._children
+        findComponent(componentIds, screenChildren)
+      }
     }
   } else {
     // It's a page, so set the screentype to page.