Referral to a non-existing plan should throw a warning rather than crashing with NPE

Step to replicate

1. Create a sample plan
2. Verify that the plan is accessible through the browser
3. Delete the plan and try to access the deleted plan again and you will be greeted with an NPE below:

   java.lang.NullPointerException
   	at com.atlassian.bamboo.ww2.actions.build.admin.config.ConfigureBuildDetails.doDefault(ConfigureBuildDetails.java:57)
   	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:597)
   	at com.opensymphony.xwork.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:358)
   	at com.opensymphony.xwork.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:218)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:192)
   	at com.opensymphony.xwork.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:88)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.opensymphony.xwork.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:88)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.atlassian.bamboo.ww2.interceptors.JSONValidationInterceptor.doIntercept(JSONValidationInterceptor.java:68)
   	at com.atlassian.bamboo.ww2.interceptors.AbstractBambooInterceptor$1.call(AbstractBambooInterceptor.java:36)
   	at com.atlassian.bamboo.ww2.interceptors.AbstractBambooInterceptor$1.call(AbstractBambooInterceptor.java:32)
   	at com.atlassian.bamboo.util.BambooProfilingUtils.withUtilTimerStack(BambooProfilingUtils.java:49)
   	at com.atlassian.bamboo.ww2.interceptors.AbstractBambooInterceptor.intercept(AbstractBambooInterceptor.java:31)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.atlassian.bamboo.ww2.interceptors.SimpleConversionErrorInterceptor.intercept(SimpleConversionErrorInterceptor.java:78)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:31)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.opensymphony.webwork.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:174)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:31)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
   	at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:31)
   	at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)