avoid trying to refresh viewpager from wrong thread, or with invalid activity

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

@@ -557,7 +557,11 @@ class MainActivity : SimpleActivity(), RefreshRecyclerViewListener {
     }
 
     private fun refreshViewPager() {
-        currentFragments.last().refreshEvents()
+        if (!isActivityDestroyed()) {
+            runOnUiThread {
+                currentFragments.last().refreshEvents()
+            }
+        }
     }
 
     private fun tryImportEvents() {

app/src/main/kotlin/com/simplemobiletools/calendar/fragments/MyFragmentHolder.kt

@@ -1,7 +1,6 @@
 package com.simplemobiletools.calendar.fragments
 
 import android.support.v4.app.Fragment
-import org.joda.time.DateTime
 
 abstract class MyFragmentHolder : Fragment() {
     abstract fun goToToday()