fix(sort-prop-types): Fix sortShapeProp when shape is not an object literal

Author: justinanastos

lib/rules/sort-prop-types.js

@@ -77,6 +77,12 @@ module.exports = {
      * @returns {void}
      */
     function checkSorted(declarations) {
+      // Declarations will be `undefined` if the `shape` is not a literal. For
+      // example, if it is a propType imported from another file.
+      if (!declarations) {
+        return;
+      }
+
       declarations.reduce((prev, curr, idx, decls) => {
         if (/SpreadProperty$/.test(curr.type)) {
           return decls[idx + 1];