JSpecify: handling the return of a diamond operator anonymous object method caller (#858)

In reference to the
[exception](https://github.com/uber/NullAway/issues/791#issuecomment-1803020173)
mentioned in the discussion for #791 .

Adding the test case to reproduce the same issue:

```java
class Test {
    static class B<T>{
        String build(){return "x";}
    }
    static String testNegative() {
        //We were getting the aforementioned mentioned exception when we tried to do this
        return new B<>().build();
    }
}
```
All unit tests have passed for the changes that were made for this.

---------

Co-authored-by: Manu Sridharan <msridhar@gmail.com>

2 files changed, 25 insertions, 4 deletions

diff --git a/nullaway/src/main/java/com/uber/nullaway/GenericsChecks.java b/nullaway/src/main/java/com/uber/nullaway/GenericsChecks.java
index ef56490..4870366 100644
--- a/nullaway/src/main/java/com/uber/nullaway/GenericsChecks.java
+++ b/nullaway/src/main/java/com/uber/nullaway/GenericsChecks.java
@@ -788,10 +788,13 @@ public final class GenericsChecks {
       return Nullness.NONNULL;
     }
     Type methodReceiverType =
-        castToNonNull(
-            getTreeType(((MemberSelectTree) tree.getMethodSelect()).getExpression(), state));
-    return getGenericMethodReturnTypeNullness(
-        invokedMethodSymbol, methodReceiverType, state, config);
+        getTreeType(((MemberSelectTree) tree.getMethodSelect()).getExpression(), state);
+    if (methodReceiverType == null) {
+      return Nullness.NONNULL;
+    } else {
+      return getGenericMethodReturnTypeNullness(
+          invokedMethodSymbol, methodReceiverType, state, config);
+    }
   }
 
   /**
diff --git a/nullaway/src/test/java/com/uber/nullaway/NullAwayJSpecifyGenericsTests.java b/nullaway/src/test/java/com/uber/nullaway/NullAwayJSpecifyGenericsTests.java
index 58f09d1..eb7c02f 100644
--- a/nullaway/src/test/java/com/uber/nullaway/NullAwayJSpecifyGenericsTests.java
+++ b/nullaway/src/test/java/com/uber/nullaway/NullAwayJSpecifyGenericsTests.java
@@ -1522,6 +1522,24 @@ public class NullAwayJSpecifyGenericsTests extends NullAwayTestsBase {
         .doTest();
   }
 
+  @Test
+  public void testForDiamondOperatorReturnedAsAMethodCaller() {
+    makeHelper()
+        .addSourceLines(
+            "Test.java",
+            "package com.uber;",
+            "import org.jspecify.annotations.Nullable;",
+            "class Test {",
+            "  static class B<T>{",
+            "   String build(){return \"x\";}",
+            "  }",
+            "  static String testNegative() {",
+            "   return new B<>().build();",
+            "  }",
+            "}")
+        .doTest();
+  }
+
   private CompilationTestHelper makeHelper() {
     return makeTestHelperWithArgs(
         Arrays.asList(