Fixes issue 3642: Invalid URLs for background images causes an exception

Change-Id: I080acad5cc07e7b1a6dd98f56e3b5d4ee3f45882
Reviewed-on: http://gerrit.dmdirc.com/659
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>
Automatic-Compile: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/addons/ui_swing/components/desktopPane/DMDircDesktopPane.java

 import java.beans.PropertyChangeEvent;
 import java.beans.PropertyChangeListener;
 import java.io.IOException;
+import java.net.URL;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
             @Override
             public void run() {
                 try {
-                    backgroundImage = ImageIO.read(URLBuilder.buildURL(
-                            backgroundPath));
+                    final URL url = URLBuilder.buildURL(backgroundPath);
+                    if (url != null) {
+                        backgroundImage = ImageIO.read(url);
+                    }
                 } catch (IOException ex) {
                     backgroundImage = null;
                 }

src/com/dmdirc/addons/ui_swing/textpane/TextPaneCanvas.java

 import java.awt.font.TextLayout;
 import java.awt.image.BufferedImage;
 import java.io.IOException;
+import java.net.URL;
 import java.text.AttributedCharacterIterator;
 import java.text.AttributedString;
 import java.util.HashMap;
             @Override
             public void run() {
                 try {
-                    backgroundImage = ImageIO.read(URLBuilder.
-                            buildURL(backgroundPath));
+                    final URL url = URLBuilder.buildURL(backgroundPath);
+                    if (url != null) {
+                        backgroundImage = ImageIO.read(url);
+                    }
                 } catch (IOException ex) {
                     backgroundImage = null;
                 }