Prevent NPE when a SCSS file is not found #11099

Change-Id: Id0e0fd6a31f4089228b02bf8b66086d3f3a1727b

theme-compiler/src/com/vaadin/sass/SassCompiler.java

 package com.vaadin.sass;
 
 import java.io.File;
+import java.io.FileNotFoundException;
 import java.io.FileWriter;
 import java.io.IOException;
 
         // ScssStylesheet.setStylesheetResolvers(new VaadinResolver());
 
         ScssStylesheet scss = ScssStylesheet.get(input);
+        if(scss == null){
+            System.err.println("The scss file " + input
+                    + " could not be found.");
+            return;
+        }
+        
         scss.compile();
         if (output == null) {
             System.out.println(scss.toString());

theme-compiler/src/com/vaadin/sass/internal/ScssStylesheet.java

      * ScssStylesheet tree out of it. Calling compile() on it will transform
      * SASS into CSS. Calling toString() will print out the SCSS/CSS.
      * 
-     * @param file
+     * @param identifier
+     *            The file path. If null then null is returned.
      * @return
      * @throws CSSException
      * @throws IOException
      * builds up a ScssStylesheet tree out of it. Calling compile() on it will
      * transform SASS into CSS. Calling toString() will print out the SCSS/CSS.
      * 
-     * @param file
+     * @param identifier
+     *            The file path. If null then null is returned.
      * @param encoding
      * @return
      * @throws CSSException
          * 
          * @charset declaration, the default one is ASCII.
          */
+
+        if (identifier == null) {
+            return null;
+        }
+
+        // FIXME Is this actually intended? /John 1.3.2013
         File file = new File(identifier);
         file = file.getCanonicalFile();