Fix bug, about application collapse by

db can't connect

src/db/SQLHelper.cpp

@@ -21,7 +21,12 @@ bool SQLHelper::Open() {
     defaultDatabase->setDatabaseName("USCT");  // 数据库名称
     defaultDatabase->setUserName("rc");  // 数据库用户名
     defaultDatabase->setPassword("123$%^");  // 数据库密码
-    return defaultDatabase->open();
+    bool ret = defaultDatabase->open();
+    if (!ret){
+        delete defaultDatabase;
+        defaultDatabase = nullptr;
+    }
+    return ret;
 }
 
 bool SQLHelper::Open(QSqlDatabase *base) {