Fix

app/Http/Livewire/CalendarRemove.php

@@ -42,7 +42,7 @@ class CalendarRemove extends Component
 
                 $hour = date("H:i", strtotime($calendar->from));
 
-                $x = $calendar->course->name . " " . $d . " " . $hour;
+                $x = ($calendar->course ? ($calendar->course->name . " ") : "") . $d . " " . $hour;
                 
                 if (!in_array($x, $this->groups))
                     $this->groups[] = $x;