Fix null warnings in AboutBox1

One warning was left because it should not ever return null unless the
package has been damaged.
All other warnings have been resolved. This should close issue #4.

AboutBox1.cs

@@ -37,7 +37,7 @@ namespace PDF_Merge
                         return titleAttribute.Title;
                     }
                 }
-                return Path.GetFileNameWithoutExtension(Assembly.GetExecutingAssembly().CodeBase);
+                return Path.GetFileNameWithoutExtension(Assembly.GetExecutingAssembly().Location);
             }
         }
 
@@ -45,7 +45,7 @@ namespace PDF_Merge
         {
             get
             {
-                return Assembly.GetExecutingAssembly().GetName().Version.ToString();
+                return Assembly.GetExecutingAssembly()?.GetName()?.Version?.ToString();
             }
         }