Avoid swipe related crash

app/src/main/kotlin/com/tutpro/baresip/OnSwipeTouchListener.kt

@@ -33,8 +33,9 @@ open class OnSwipeTouchListener(ctx: Context) : View.OnTouchListener {
 
         override fun onFling(e1: MotionEvent?, e2: MotionEvent?, velX: Float, velY: Float): Boolean {
             var result = false
+            if (e1 != null && e2 != null) {
                 try {
-                val diffY = e2!!.y - e1!!.y
+                    val diffY = e2.y - e1.y
                     val diffX = e2.x - e1.x
                     if (abs(diffX) > abs(diffY)) {
                         if (abs(diffX) > SWIPE_THRESHOLD && abs(velX) > SWIPE_VELOCITY_THRESHOLD) {
@@ -58,7 +59,7 @@ open class OnSwipeTouchListener(ctx: Context) : View.OnTouchListener {
                     Log.e(TAG, "onFling exception $e")
                     e.printStackTrace()
                 }
-
+            }
             return result
         }