Fix a crash in VarChecker on effectively final resource variables

The final modifier appears in the AST even if it was implicit in the
source.

Fixes #498

MOE_MIGRATED_REVID=149767713

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

@@ -89,10 +89,11 @@ private Description handleLocalOrParam(VariableTree tree, VisitorState state, Sy
       if (Source.instance(state.context).allowEffectivelyFinalInInnerClasses()) {
         // In Java 8, the final modifier is never necessary on locals/parameters because
         // effectively final variables can be used anywhere a final variable is required.
-        return buildDescription(tree)
-            .setMessage(UNNECESSARY_FINAL)
-            .addFix(SuggestedFixes.removeModifiers(tree, state, Modifier.FINAL))
-            .build();
+        Fix fix = SuggestedFixes.removeModifiers(tree, state, Modifier.FINAL);
+        // The fix may be null for TWR variables that were not explicitly final
+        if (fix != null) {
+          return buildDescription(tree).setMessage(UNNECESSARY_FINAL).addFix(fix).build();
+        }
       }
       return Description.NO_MATCH;
     }

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

@@ -310,4 +310,20 @@ public void finalTWR() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void nonFinalTWR() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import java.io.InputStream;",
+            "class Test {",
+            "  public void x() {",
+            "    try (InputStream is = null) {",
+            "    } catch (Exception e) {",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
 }