Alert Configuration link for Disk Utilization throws Error

When clicking the Alert Configuration link for the Disk Utlization of some Windows monitor instances , it is throwing error.

ErrorMsg:
java.lang.NumberFormatException: For input string: "null" at
java.lang.NumberFormatException.forInputString(Unknown Source) at
java.lang.Integer.parseInt(Unknown Source) at
java.lang.Integer.parseInt(Unknown Source) at
jsp.ThresholdActionConfiguration_jsp._jspService(ThresholdActionConfiguration_jsp.java:540)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:137)
at
javax.servlet.http.HttpServlet.service(HttpServlet.java:853) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)

Thanks