Account for missing option includes (#31648)

Fixes: #31648.
Bug:
Change-Id: I1fae4c0aeff4935ca212b3d9ef52645492f7465c
Reviewed-on: https://dart-review.googlesource.com/29500
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

pkg/analyzer/lib/src/task/options.dart

@@ -281,7 +281,7 @@ class GenerateOptionsErrorsTask extends SourceBasedAnalysisTask {
       initialIncludeSpan ??= span;
       String includeUri = span.text;
       Source includedSource = sourceFactory.resolveUri(source, includeUri);
-      if (!includedSource.exists()) {
+      if (includedSource == null || !includedSource.exists()) {
         errors.add(new AnalysisError(
             initialSource,
             initialIncludeSpan.start.column + 1,