VariableRequiresSetAssignmentEvaluator is missing a check for null

[MDoerner]

When pasing the projec from #3178, I got the following warning in the log.

2017-07-29 20:37:42.1658;WARN-2.0.14.24961;Rubberduck.Inspections.Rubberduck.Inspections.Inspector+<>c__DisplayClass6_2;System.NullReferenceException: Object reference not set to an instance of an object.
   at Rubberduck.Inspections.VariableRequiresSetAssignmentEvaluator.RHSUsesNew(LetStmtContext letStmtContext)
   at Rubberduck.Inspections.VariableRequiresSetAssignmentEvaluator.ObjectOrVariantRequiresSetAssignment(IdentifierReference objectOrVariantRef, IEnumerable`1 variantAndObjectDeclarations)
   at Rubberduck.Inspections.VariableRequiresSetAssignmentEvaluator.RequiresSetAssignment(IdentifierReference reference, RubberduckParserState state)
   at Rubberduck.Inspections.Concrete.ObjectVariableNotSetInspection.<GetInspectionResults>b__3_3(IdentifierReference reference)
   at System.Linq.Enumerable.<>c__DisplayClass6_0`1.<CombinePredicates>b__0(TSource x)
   at System.Linq.Enumerable.<>c__DisplayClass6_0`1.<CombinePredicates>b__0(TSource x)
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Rubberduck.Inspections.Rubberduck.Inspections.Inspector.<>c__DisplayClass6_2.<FindIssuesAsync>b__5();

Apparently, a check for null is missing somewhere in the VariableRequiresSetAssignmentEvaluator. Maybe something else is wrong, too.

[retailcoder]

Line 99 is assuming the parent LetStmt context won't be null, so RHSUsesNew is getting a null parameter, which throws the exception when we try to make a member call on it. The method could use a guard clause (and return false given a null parameter), and/or caller (line 99) needs to check if letStmt is null. That said, how that context can be null is an interesting thing to look into (i.e. there would be no LetStmtContext in the parent nodes, ...which makes me wonder how that node even got there).

[MDoerner]

I am currently looking at the inspection. One thing I can see already is that it seems to assume that assignments are always a letStmt or a setStmt, which is not true.