【APPCenter】Crash编号：6（KMImageToolTipWindow）

PDF Office/PDF Master/Class/PDFWindowController/PDFListView/CPDFKitExtensions/CPDFAnnotationExtensions/CPDFLinkAnnotation+PDFListView.swift

@@ -125,7 +125,12 @@ import Foundation
     }
     
     override func linkURL() -> URL? {
-        return URL(string: self.url())
+        let urlStr = self.url()
+        if urlStr != nil {
+            return URL(string: self.url())
+        } else {
+            return URL(string: "")
+        }
     }
     
     func isSkimNote() -> Bool {