Check for null before dereferencing constraint text.

Bug: #38839
Change-Id: I63104ec32d871a61f7e5f65e605ea01faf0d2aaf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124464
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>
Auto-Submit: Janice Collins <jcollins@google.com>

Author: jcollins-g

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

@@ -4376,10 +4376,10 @@ class FixProcessor extends BaseProcessor {
     SdkConstraintExtractor extractor = new SdkConstraintExtractor(pubspecFile);
     String text = extractor.constraintText();
     int offset = extractor.constraintOffset();
-    int length = text.length;
     if (text == null || offset < 0) {
       return;
     }
+    int length = text.length;
     String newText;
     int spaceOffset = text.indexOf(' ');
     if (spaceOffset >= 0) {