Fix event add modal handler registration

static/app.js

@@ -85,7 +85,10 @@
     })
   }
 
-  document.getElementById("add-event-modal").addEventListener("show.bs.modal", e => {
+  const addEventModal = document.getElementById("add-event-modal")
-    document.getElementById("event").value = e.relatedTarget.dataset.eventType
+  if (addEventModal !== null) {
-  })
+    addEventModal.addEventListener("show.bs.modal", e => {
+      document.getElementById("event").value = e.relatedTarget.dataset.eventType
+    })
+  }
 })()