make fragment click listener nullable

app/src/main/kotlin/com/simplemobiletools/gallery/fragments/PhotoFragment.kt

@@ -66,6 +66,6 @@ class PhotoFragment : ViewPagerFragment(), View.OnClickListener {
         if (listener == null)
             listener = activity as ViewPagerFragment.FragmentClickListener
 
-        listener.fragmentClicked()
+        listener?.fragmentClicked()
     }
 }

app/src/main/kotlin/com/simplemobiletools/gallery/fragments/VideoFragment.kt

@@ -161,7 +161,11 @@ class VideoFragment : ViewPagerFragment(), View.OnClickListener, SurfaceHolder.C
             else -> {
                 mIsFullscreen = !mIsFullscreen
                 checkFullscreen()
-                listener.fragmentClicked()
+
+                if (listener == null)
+                    listener = activity as ViewPagerFragment.FragmentClickListener
+
+                listener?.fragmentClicked()
             }
         }
     }

app/src/main/kotlin/com/simplemobiletools/gallery/fragments/ViewPagerFragment.kt

@@ -3,7 +3,7 @@ package com.simplemobiletools.gallery.fragments
 import android.support.v4.app.Fragment
 
 abstract class ViewPagerFragment : Fragment() {
-    lateinit var listener: FragmentClickListener
+    var listener: FragmentClickListener? = null
 
     abstract fun itemDragged()