Fix null warnings in AboutBox1

One warning was left because it should not ever return null unless the
package has been damaged.

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();
             }
         }