web/DialogContainer: prevent an error after a race condition

an error is no longer thrown if several dialogs were closed while timeout was running

this should really be replaced by proper dialog management system, with each dialog having a unique id and removal happening via that id, not just array.pop()

web/src/components/dialog/DialogContainer.svelte

@@ -16,9 +16,14 @@
         if (dialogParent) {
             closing = true;
             open = false;
+
+            // wait 150ms for the closing animation to finish
             setTimeout(() => {
-                dialogParent.close();
-                killDialog();
+                // check if dialog parent is still present
+                if (dialogParent) {
+                    dialogParent.close();
+                    killDialog();
+                }
             }, 150);
         }
     };