fix: close https://gitee.com/mybatis-flex/mybatis-flex/issues/IBGNNF

mybatis-flex-core/src/main/java/com/mybatisflex/core/datasource/DataSourceKey.java

@@ -33,20 +33,26 @@ public class DataSourceKey {
     }
 
     public static void use(String dataSourceKey) {
-        lookup.get().push(dataSourceKey);
+        Deque<String> deque = lookup.get();
+        if (deque != null) {
+            deque.push(dataSourceKey);
+        }
     }
 
     public static String get() {
-        return lookup.get().peek();
+        Deque<String> deque = lookup.get();
+        return deque != null ? deque.peek() : null;
     }
 
     public static void clear() {
         Deque<String> deque = lookup.get();
+        if (deque != null) {
             deque.pop();
             if (deque.isEmpty()) {
                 lookup.remove();
             }
         }
+    }
 
     public static void forceClear() {
         lookup.remove();
@@ -89,18 +95,22 @@ public class DataSourceKey {
     public static String getByManual() {
         throw new UnsupportedOperationException("使用 DataSource.get() 代替。");
     }
+
     @Deprecated
     public static String getByAnnotation() {
         throw new UnsupportedOperationException("使用 DataSource.get() 代替。");
     }
+
     @Deprecated
     public static void useWithAnnotation(String dataSourceKey) {
         throw new UnsupportedOperationException("使用 DataSource.use(String) 代替。");
     }
+
     @Deprecated
     public static void setAnnotationKeyThreadLocal(ThreadLocal<String> annotationKeyThreadLocal) {
         throw new UnsupportedOperationException("使用 DataSource.setThreadLocal(ThreadLocal<Deque<String>>) 代替。");
     }
+
     @Deprecated
     public static void setManualKeyThreadLocal(ThreadLocal<String> manualKeyThreadLocal) {
         throw new UnsupportedOperationException("使用 DataSource.setThreadLocal(ThreadLocal<Deque<String>>) 代替。");