show a message if theres no events found for exporting

app/src/main/kotlin/com/simplemobiletools/calendar/activities/MainActivity.kt

@@ -264,6 +264,11 @@ class MainActivity : SimpleActivity(), NavigationListener {
             ExportEventsDialog(this, path) {
                 Thread({
                     val events = dbHelper.getEventsToExport(it)
+                    if (events.isEmpty()) {
+                        runOnUiThread {
+                            toast(R.string.no_events_for_exporting)
+                        }
+                    } else {
                         val result = IcsExporter().exportEvents(this, path, events)
                         runOnUiThread {
                             toast(when (result) {
@@ -272,6 +277,7 @@ class MainActivity : SimpleActivity(), NavigationListener {
                                 else -> R.string.exporting_events_failed
                             })
                         }
+                    }
                 }).start()
             }
         }