GetCharTable throws KeyNotFoundException

When a Unicode character is encountered that is not included in the default conversion code tables, a call to ReverseCodeTableHash.GetCharTable() throws a KeyNotFoundException.

The only method that calls GetCharTable is ReverseCodeTable.CodeTableHash()

Looking at usage of CodeTableHash, especially in ReverseCodeTable.CharHasMatch, it appears the correct outcome when a char is not found is to return null.

All other uses (except one) of CodeTableHash include a check for null on the returned value.