freemarker.runtime : Error executing FreeMarker template
freemarker.core.InvalidReferenceException: The following has evaluated to null or missing:
==> courses [in template "layout.ftl" at line 62, column 40]
layout.ftl 文件 62 行变量 courses 值为 null 导致，可以检查下代码，保证变量值不能为 null。