Fix error on checking the initTree2PrevFieldInit cache. (#252)

We were saving a `ClassSymbol` but testing for the `ClassTree`. 
This didn't cause a functional problem, but it means the cache wasn't being hit.
diff --git a/nullaway/src/main/java/com/uber/nullaway/NullAway.java b/nullaway/src/main/java/com/uber/nullaway/NullAway.java
index 393af86..7cab42f 100644
--- a/nullaway/src/main/java/com/uber/nullaway/NullAway.java
+++ b/nullaway/src/main/java/com/uber/nullaway/NullAway.java
@@ -942,7 +942,8 @@
       Symbol fieldSymbol, TreePath initTreePath, VisitorState state) {
     TreePath enclosingClassPath = initTreePath.getParentPath();
     ClassTree enclosingClass = (ClassTree) enclosingClassPath.getLeaf();
-    Multimap<Tree, Element> tree2Init = initTree2PrevFieldInit.get(enclosingClass);
+    Multimap<Tree, Element> tree2Init =
+        initTree2PrevFieldInit.get(ASTHelpers.getSymbol(enclosingClass));
     if (tree2Init == null) {
       tree2Init = computeTree2Init(enclosingClassPath, state);
       initTree2PrevFieldInit.put(ASTHelpers.getSymbol(enclosingClass), tree2Init);
diff --git a/nullaway/src/main/java/com/uber/nullaway/handlers/InferredJARModelsHandler.java b/nullaway/src/main/java/com/uber/nullaway/handlers/InferredJARModelsHandler.java
index 4d03f12..44f1087 100644
--- a/nullaway/src/main/java/com/uber/nullaway/handlers/InferredJARModelsHandler.java
+++ b/nullaway/src/main/java/com/uber/nullaway/handlers/InferredJARModelsHandler.java
@@ -238,9 +238,6 @@
           // Load model jars
           for (String modelJarPath : mapModelJarLocations.get(jarName)) {
             JarFile jar = new JarFile(modelJarPath);
-            if (jar == null) {
-              throw new Error("Cannot open jar: " + modelJarPath);
-            }
             LOG(DEBUG, "DEBUG", "Found model jar at: " + modelJarPath);
             JarEntry astubxJE = jar.getJarEntry(DEFAULT_ASTUBX_LOCATION);
             if (astubxJE == null) {