Fix a crash in YodaCondition

core/src/main/java/com/google/errorprone/bugpatterns/YodaCondition.java

@@ -79,12 +79,12 @@ public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState
           state);
     }
     if (instanceEqualsInvocation().matches(tree, state)) {
-      return fix(
-          tree,
-          getReceiver(tree),
-          tree.getArguments().get(0),
-          /* provideNullSafeFix= */ true,
-          state);
+      ExpressionTree receiver = getReceiver(tree);
+      if (receiver == null) {
+        // call to equals implicitly qualified by `this`
+        return NO_MATCH;
+      }
+      return fix(tree, receiver, tree.getArguments().get(0), /* provideNullSafeFix= */ true, state);
     }
     return NO_MATCH;
   }

core/src/test/java/com/google/errorprone/bugpatterns/YodaConditionTest.java

@@ -251,4 +251,22 @@ public void nullableYodaCondition() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void unqualified() {
+    testHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.common.base.Objects;",
+            "class Test {",
+            "  @Override",
+            "  public boolean equals(Object other) {",
+            "    return Objects.equal(this, other);",
+            "  }",
+            "  public boolean foo(Object other) {",
+            "    return equals(other);",
+            "  }",
+            "}")
+        .doTest();
+  }
 }