Fix detecting of non-physical files in ClassFileFactory

compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java

@@ -272,6 +272,7 @@ public class ClassFileFactory implements OutputFileCollection {
     private static List<File> toIoFilesIgnoringNonPhysical(@NotNull Collection<? extends PsiFile> psiFiles) {
         List<File> result = new ArrayList<>(psiFiles.size());
         for (PsiFile psiFile : psiFiles) {
+            if (psiFile == null) continue;
             VirtualFile virtualFile = psiFile.getVirtualFile();
             // We ignore non-physical files here, because this code is needed to tell the make what inputs affect which outputs
             // a non-physical file cannot be processed by make