flowable-idm: HTTP 500 – When user was deleted but tries to access flowable UI application with existing cookie

When user was deleted from IDM, but has a valid cookie he gets an HTTP 500 Error when he tries to access a flowable application:

```
HTTP Status 500 – Internal Server Error

Type Exception Report

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

java.lang.NullPointerException
com.google.common.base.Preconditions.checkNotNull(Preconditions.java:213)
com.google.common.cache.LocalCache.get(LocalCache.java:3933)
com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:3938)
com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4821)
org.flowable.app.idm.service.UserCacheImpl.getUser(UserCacheImpl.java:104)
org.flowable.app.security.UserDetailsService.loadByUserId(UserDetailsService.java:89)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:498)
org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:333)
org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190)
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:282)
org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:213)
com.sun.proxy.$Proxy44.loadByUserId(Unknown Source)
org.flowable.app.security.CustomPersistentRememberMeServices.processAutoLoginCookie(CustomPersistentRememberMeServices.java:144)
org.springframework.security.web.authentication.rememberme.AbstractRememberMeServices.autoLogin(AbstractRememberMeServices.java:130)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:498)
org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:333)
org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:207)
com.sun.proxy.$Proxy46.autoLogin(Unknown Source)
org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:98)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:170)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:200)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:116)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:64)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:105)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:56)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:214)
org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:177)
org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:262)
```

Start at org.flowable.app.idm.service.UserCacheImpl.getUser(UserCacheImpl.java:104), then follow UserDetailsService.loadByUserId and CustomPersistentRememberMeServices.processAutoLoginCookie in the stack trace. Reproduce access with a valid cookie after deleting the user from IDM, and verify that this case no longer results in the reported HTTP 500 error.

Written by the indexing model from the issue text.