Initial problem in opening model after creating and immediately deleting specific multirelationship in specific model

[MartinFijneman]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have verified that I am running the latest version of the COMET-IME
• [X] I have searched open and closed issues to ensure it has not already been reported

Description

model name: Space Demo Copy with 5 iterations - version 2 model shortname: SD_COPY_2

Not able to open engineering model anymore after creating and deleting specific multirelationship after initial closing of model.

problem seems to occur with specific model and testing steps, but is reproducible for me (with the items given below).

need to disconnect from server and reconnect, then it is possible to open the model again.

(Note: issue noticed while testing #850 )

Steps to Reproduce

• open model on dev-server
• create multirelationship between element usage "SS_PWR_RS" (top of the list of element usages), and requirement "Req2" (see picture below)

• delete this multirelationship again
• close engineering model
• try to open engineering model > Object reference error, see details below

Solution: Timeboxed!

• Try to reproduce and find out what exactly is wrong.
• Create a unit test in SDK to reproduce it and fix it from there.

Logs

• [ ] There are no logs regarding the issue
• [X] I have attached a log file or included the relevant section in this issue

Error message in the IME log browser:

Error

System.NullReferenceException: Object reference not set to an instance of an object.
   at CDP4Common.Helpers.PocoThingFactory.Get[T](ConcurrentDictionary`2 cache, CacheKey key)
   at CDP4Common.Helpers.PocoThingFactory.Get[T](ConcurrentDictionary`2 cache, Guid itemIid, Nullable`1 iterationId)
   at CDP4Common.Helpers.PocoThingFactory.TryGet[T](ConcurrentDictionary`2 cache, Guid itemIid, Nullable`1 iterationId, T& thing)
   at CDP4Common.Helpers.PocoThingFactory.ResolveList[T](List`1 list, IEnumerable`1 guidList, Nullable`1 iterationId, ConcurrentDictionary`2 cache)
   at CDP4Common.SiteDirectoryData.Category.ResolveProperties(Thing dtoThing)
   at CDP4Common.Helpers.PocoThingFactory.ResolveDependencies(IEnumerable`1 dtoThings, ConcurrentDictionary`2 cache)
   at CDP4Dal.Assembler.<Synchronize>d__15.MoveNext()  

### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->

- COMET version:
  - [X] COMET-IME: 9.3.0 RC1
  - [ ] Other: 
  -
- Web Services:
  - [ ] CDP4/COMET Web Services version: 
  - [ ] OCDT WSP version: 
- Environment (Operating system, version and so on):
  - [ ] Microsoft Excel (32/64 bit): 
  - [ ] Windows OS (32/64 bit): 
  - [ ] Additional information: