fix #87, do not show "No valid app found" at sharing

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

@@ -365,16 +365,19 @@ class MainActivity : SimpleActivity(), CanvasListener {
         val newPath = "$folder/$FILE_NAME"
         val fileDirItem = FileDirItem(newPath, FILE_NAME)
         getFileOutputStream(fileDirItem, true) {
+            if (it != null) {
                 try {
-                it?.write(bytes.toByteArray())
+                    it.write(bytes.toByteArray())
                     callback(newPath)
                 } catch (e: Exception) {
                 } finally {
-                it?.close()
-            }
+                    it.close()
                 }
+            } else {
                 callback("")
             }
+        }
+    }
 
     private fun clearCanvas() {
         my_canvas.clearCanvas()