• In JIRA WAR/Webapp distribution, copy the class to edit-webapp/WEB-INF/classes/com/atlassian/jira/issue/managers/DefaultIssueManager.class
  (creating intermediate directories as necessary), and rebuild the .war
• In JIRA Standalone, copy to atlassian-jira/WEB-INF/classes/com/atlassian/jira/issue/managers/DefaultIssueManager.class
  and restart (bin/shutdown, bin/startup).

After this I applied the patch you have created and Know I have another error:

Select Mantis projects to import
Step 2 of 3: Select Mantis projects to import Form Errors:
Error importing data from Mantis.java.sql.SQLException: Illegal operation on empty result set at com.mysql.jdbc.ResultSet.checkRowPos(ResultSet.java:3623) at com.mysql.jdbc.ResultSet.getString(ResultSet.java:1767) at com.mysql.jdbc.ResultSet.getString(ResultSet.java:1831) at com.atlassian.jira.util.MantisImportBean.getDescription(MantisImportBean.java:418) at com.atlassian.jira.util.MantisImportBean.importIssue(MantisImportBean.java:338) at com.atlassian.jira.util.MantisImportBean.importIssues(MantisImportBean.java:307) at com.atlassian.jira.util.MantisImportBean.importProjects(MantisImportBean.java:763) at com.atlassian.jira.util.MantisImportBean.doImport(MantisImportBean.java:185) at com.atlassian.jira.web.action.util.MantisImport.doRun(MantisImport.java:136) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:324) at webwork.action.ActionSupport.invokeCommand(ActionSupport.java:415) at webwork.action.ActionSupport.execute(ActionSupport.java:144) at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:49) at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:131) at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:186) at javax.servlet.http.HttpServlet.service(HttpServlet.java:856) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:284) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:129) at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:61) at com.atlassian.jira.web.filters.SitemeshExcludePathFilter.doFilter(SitemeshExcludePathFilter.java:36) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:164) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:181) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.jira.web.filters.ActionCleanupDelayFilter.doFilter(ActionCleanupDelayFilter.java:37) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.jira.web.filters.gzip.GzipFilter.doFilter(GzipFilter.java:60) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:38) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:233) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:204) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:257) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:151) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:567) at org.apache.catalina.core.StandardContextValve.invokeInternal(StandardContextValve.java:245) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:199) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:151) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:567) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:184) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:151) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:164) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:149) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:578) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:149) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:567) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:156) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:151) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:567) at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:972) at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:206) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:833) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:732) at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:619) at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:688) at java.lang.Thread.run(Thread.java:534)