Fix insert image file picker only opens once

When no image is selected from the intent we should set onReceiveValue to null and uploadMessage to null otherwise it does not open after closing it without selecting an image.

src/main/java/com/owncloud/android/ui/activity/RichDocumentsEditorWebView.java

@@ -144,7 +144,10 @@ public class RichDocumentsEditorWebView extends EditorWebView {
     @Override
     protected void onActivityResult(int requestCode, int resultCode, Intent data) {
         if (RESULT_OK != resultCode) {
-            // TODO
+            if (requestCode == REQUEST_LOCAL_FILE) {
+                this.uploadMessage.onReceiveValue(null);
+                this.uploadMessage = null;
+            }
             return;
         }