SUBMARINE-1149. Sonar: Fix reliability issues in submarine-server

Signed-off-by: Kevin Su <pingsutwapache.org>

### What is this PR for?
<!-- A few sentences describing the overall goals of the pull request's commits.
First time? Check out the contributing guide - https://submarine.apache.org/contribution/contributions.html
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Try to fix the issues on the sonarcloud. https://sonarcloud.io/project/issues?id=apache_submarine&resolved=false&sinceLeakPeriod=true&types=BUG

### What type of PR is it?
[Bug Fix]

### Todos
* [ ] - Task

### What is the Jira issue?
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* Put link here, and add [SUBMARINE-*Jira number*] in PR title, eg. `SUBMARINE-23. PR title`
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https://issues.apache.org/jira/browse/SUBMARINE-1149

### How should this be tested?
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* Strongly recommended: add automated unit tests for any new or changed behavior
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pass the cis
### Screenshots (if appropriate)

### Questions:
* Do the license files need updating? No
* Are there breaking changes for older versions? No
* Does this need new documentation? No

Author: Kevin Su <pingsutw@apache.org>

Signed-off-by: Kevin <pingsutw@apache.org>

Closes apache#841 from pingsutw/SUBMARINE-1149 and squashes the following commits:

6551936 [Kevin Su] Sonar: Fix reliability issues in submarine-server

submarine-server/server-core/src/main/java/org/apache/submarine/server/model/ModelManager.java

@@ -36,7 +36,7 @@
 public class ModelManager {
   private static final Logger LOG = LoggerFactory.getLogger(ModelManager.class);
 
-  private static volatile ModelManager manager;
+  private static ModelManager manager;
 
   private final Submitter submitter;
 

submarine-server/server-core/src/main/java/org/apache/submarine/server/workbench/database/utils/MybatisGenerator.java

@@ -47,27 +47,26 @@ public static void main(String[] args) {
     Configuration config = null;
     try {
       config = cp.parseConfiguration(configFile);
-    } catch (IOException e) {
-      LOG.error(e.getMessage(), e);
-    } catch (XMLParserException e) {
+    } catch (IOException | XMLParserException e) {
       LOG.error(e.getMessage(), e);
     }
 
     DefaultShellCallback callback = new DefaultShellCallback(overwrite);
     MyBatisGenerator myBatisGenerator = null;
     try {
+      assert config != null;
       myBatisGenerator = new MyBatisGenerator(config, callback, warnings);
     } catch (InvalidConfigurationException e) {
       LOG.error(e.getMessage(), e);
     }
     try {
+      assert myBatisGenerator != null;
       myBatisGenerator.generate(null);
-    } catch (SQLException e) {
-      LOG.error(e.getMessage(), e);
-    } catch (IOException e) {
+    } catch (SQLException | IOException e) {
       LOG.error(e.getMessage(), e);
     } catch (InterruptedException e) {
       LOG.error(e.getMessage(), e);
+      throw new RuntimeException(e);
     }
   }
 }