tagger analyzer not functiong properly 
**Describe the bug**
After upgrade TimeSketch to version: 20210602 the tagger analyzer is not functioning with custom tags

**To Reproduce**
Steps to reproduce the behavior:
1. Import plaso file with evtx data 
2. Add the following tagging rule to tags.yaml
```yaml
logon_tagger:  
  query_string: 'data_type: "windows:evtx:record" AND source_name: "Microsoft-Windows-Security-Auditing" AND event_identifier: 4688'
  tags: ['logon']
  save_search: true
  search_name: 'logon'
```
3. run tagger analyzer
4. See error

**Expected behavior**
The tagger analyzer to run correctly as in previous versions.

**Desktop (please complete the following information):**
-OS:Ubuntu 20.04.2 LTS
-Browser : Firefox
-Version: 86.0

**Additional context**
The following exception is thrown once the tagger analyzer is ran:
```
Traceback (most recent call last): File "/usr/local/lib/python3.8/dist-packages/timesketch/lib/analyzers/interface.py", line 995, in run_wrapper result = self.run() File "/usr/local/lib/python3.8/dist-packages/timesketch/lib/analyzers/tagger.py", line 48, in run tag_result = self.tagger(name, tag_config) File "/usr/local/lib/python3.8/dist-packages/timesketch/lib/analyzers/tagger.py", line 100, in tagger if expression: UnboundLocalError: local variable 'expression' referenced before assignment
``` 

diff --git a/timesketch/lib/analyzers/tagger.py b/timesketch/lib/analyzers/tagger.py
index 0335adf6..712489b9 100644
--- a/timesketch/lib/analyzers/tagger.py
+++ b/timesketch/lib/analyzers/tagger.py
@@ -83,6 +83,7 @@ class TaggerSketchPlugin(interface.BaseAnalyzer):
 
         expression_string = config.get('regular_expression', '')
         attributes = None
+        expression = None
         if expression_string:
             expression = utils.compile_regular_expression(
                 expression_string=expression_string,
['timesketch/lib/analyzers/tagger.py']