Description

This pr fixes the issue that when the neosystem is initialized with memstore, then called with neoSystem.StoreView;, then the commit operation will fail as the snapshot is null (snapshot = store as ISnapshot; gets null as memorystore is IStore only).

exception:

Test method Neo.UnitTests.Persistence.UT_MemoryStore.NeoSystemTest threw exception: 
System.NullReferenceException: Object reference not set to an instance of an object.
    at Neo.Persistence.SnapshotCache.Commit() in /Users/jinghuiliao/git/neo/src/Neo/Persistence/SnapshotCache.cs:line 51
   at Neo.UnitTests.Persistence.UT_MemoryStore.NeoSystemTest() in /Users/jinghuiliao/git/neo/tests/Neo.UnitTests/Persistence/UT_MemoryStore.cs:line 67
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Fixes # (issue)

Type of change

• Optimization (the change is only an optimization)
• Style (the change is only a code style for better maintenance or standard purpose)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• NeoSystemStoreViewTest

Test Configuration:

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules