AST visitors in Semantic API internal implementation throws NPEs and need to Handle them explicitly

[nadeeshaan]

In the current implementation of the semantic API, we use tree visitors to visit the AST and extract symbols. For an example, ReferenceFinder. When there are semantically invalid scenarios such as we have been added an unresolved import and when there are usages of that, Semantic API fails even for symbol extraction for the valid symbols as well. We need to check these cases properly. One such usage is
https://github.com/ballerina-platform/ballerina-lang/blob/master/compiler/ballerina-lang/src/main/java/io/ballerina/compiler/api/impl/ReferenceFinder.java#L972

Here userDefinedType.type.tsymbol is null.

@pubudu91 thoughts? What's the possibility of handle these in a common place?

As a temp fix we can skip them with a null check. But cannot guarantee that we cover all the cases gracefully