Fix NPE in ChooseTemplateDialogFragment.checkEnablingCreateButton

This method can be called before positiveButton is initialized

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

app/src/main/java/com/owncloud/android/ui/dialog/ChooseTemplateDialogFragment.java

@@ -286,11 +286,13 @@ public class ChooseTemplateDialogFragment extends DialogFragment implements View
     }
 
     private void checkEnablingCreateButton() {
-        Template selectedTemplate = adapter.getSelectedTemplate();
-        String name = binding.filename.getText().toString();
+        if (positiveButton != null) {
+            Template selectedTemplate = adapter.getSelectedTemplate();
+            String name = binding.filename.getText().toString();
 
-        positiveButton.setEnabled(selectedTemplate != null && !name.isEmpty() &&
-                                      !name.equalsIgnoreCase(DOT + selectedTemplate.getExtension()));
+            positiveButton.setEnabled(selectedTemplate != null && !name.isEmpty() &&
+                                          !name.equalsIgnoreCase(DOT + selectedTemplate.getExtension()));
+        }
     }
 
     private static class CreateFileFromTemplateTask extends AsyncTask<Void, Void, String> {