Merge pull request #756 from Netflix/fix/null-check-sourcepreview

Add null check on sourcePreview from InvalidSyntaxException

Author: paulbakker

graphql-dgs-codegen-core/src/main/kotlin/com/netflix/graphql/dgs/codegen/CodeGen.kt

@@ -139,7 +139,7 @@ class CodeGen(private val config: CodeGenConfig) {
                 parser.parseDocument(parserEnv)
             } catch (exception: InvalidSyntaxException) {
                 // check if the schema is empty
-                if (exception.sourcePreview.isBlank()) {
+                if (exception.sourcePreview == null || exception.sourcePreview.isBlank()) {
                     logger.warn("Schema is empty")
                     // return an empty document
                     return Document.newDocument().build()