[fixed] Modal Null Exception when react-bootstrap is loaded before the Body tag

Make scrollbar size check lazy, Only called after components have
mounted. fixes react-bootstrap#849

Author: jquense

src/Modal.js

@@ -54,7 +54,11 @@ function onFocus(context, handler) {
 
 let scrollbarSize;
 
-if (domUtils.canUseDom) {
+function getScrollbarSize(){
+  if ( scrollbarSize !== undefined ){
+    return scrollbarSize;
+  }
+
   let scrollDiv = document.createElement('div');
 
   scrollDiv.style.position = 'absolute';
@@ -64,13 +68,13 @@ if (domUtils.canUseDom) {
   scrollDiv.style.overflow = 'scroll';
 
   document.body.appendChild(scrollDiv);
-
   scrollbarSize = scrollDiv.offsetWidth - scrollDiv.clientWidth;
-
   document.body.removeChild(scrollDiv);
+
   scrollDiv = null;
 }
 
+
 const Modal = React.createClass({
 
   mixins: [BootstrapMixin, FadeMixin],
@@ -210,7 +214,7 @@ const Modal = React.createClass({
     this._originalPadding = container.style.paddingRight;
 
     if (this._containerIsOverflowing) {
-      container.style.paddingRight = parseInt(this._originalPadding || 0, 10) + scrollbarSize + 'px';
+      container.style.paddingRight = parseInt(this._originalPadding || 0, 10) + getScrollbarSize() + 'px';
     }
 
     if (this.props.backdrop) {
@@ -308,8 +312,8 @@ const Modal = React.createClass({
 
     return {
       dialogStyles: {
-        paddingRight: containerIsOverflowing && !modalIsOverflowing ? scrollbarSize : void 0,
-        paddingLeft:  !containerIsOverflowing && modalIsOverflowing ? scrollbarSize : void 0
+        paddingRight: containerIsOverflowing && !modalIsOverflowing ? getScrollbarSize() : void 0,
+        paddingLeft:  !containerIsOverflowing && modalIsOverflowing ? getScrollbarSize() : void 0
       }
     };
   }