Prevent export with empty password

app/src/main/java/eu/faircode/email/FragmentDialogExport.java

@@ -126,6 +126,7 @@ public class FragmentDialogExport extends FragmentDialogBase {
             @Override
             public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
                 if (actionId == EditorInfo.IME_ACTION_DONE) {
+                    if (btnOk.isEnabled())
                         btnOk.performClick();
                     return true;
                 } else