This method can return null.

I think what's going on is that GitHub takes some non-zero amount of time to compute this value, and our test runs too fast sometimes and try to fetch a PR before its mergeability is computed

src/test/java/org/kohsuke/github/PullRequestTest.java

@@ -82,7 +82,7 @@ public class PullRequestTest extends AbstractGitHubApiTestBase {
         PagedIterable<GHPullRequest> ghPullRequests = getRepository().listPullRequests(GHIssueState.OPEN);
         for (GHPullRequest pr : ghPullRequests) {
             assertNotNull(pr.getUser().root);
-            assertFalse(pr.getMergeable());
+            pr.getMergeable();
             assertNotNull(pr.getUser().root);
         }
     }