Make supsub work with empty nucleus

Auditors: emily

buildTree.js

@@ -52,7 +52,10 @@ var groupToType = {
 };
 
 var getTypeOfGroup = function(group) {
-    if (group.type === "supsub") {
+    if (group == null) {
+        // Like when typesetting $^3$
+        return groupToType.ord;
+    } else if (group.type === "supsub") {
         return getTypeOfGroup(group.value.base);
     } else if (group.type === "llap" || group.type === "rlap") {
         return getTypeOfGroup(group.value);

test/katex-tests.js

@@ -1,3 +1,4 @@
+var buildTree = require("../buildTree");
 var parseTree = require("../parseTree");
 
 describe("A parser", function() {
@@ -286,6 +287,26 @@ describe("A subscript and superscript parser", function() {
     });
 });
 
+describe("A subscript and superscript tree-builder", function() {
+    it("should not fail when there is no nucleus", function() {
+        expect(function() {
+            buildTree(parseTree("^3"));
+        }).not.toThrow();
+
+        expect(function() {
+            buildTree(parseTree("_2"));
+        }).not.toThrow();
+
+        expect(function() {
+            buildTree(parseTree("^3_2"));
+        }).not.toThrow();
+
+        expect(function() {
+            buildTree(parseTree("_2^3"));
+        }).not.toThrow();
+    });
+});
+
 describe("A group parser", function() {
     it("should not fail", function() {
         expect(function() {