修复创建文章时，初始image为空时会报错的问题

Chen-Zhitong · Chen-Zhitong · commit b6bb4cfffccc · 2020-07-14T15:44:48.000+08:00
diff --git a/src/Crop.php b/src/Crop.php
@@ -28,7 +28,8 @@ class Crop extends File
 
     protected function preview()
     {
-        return $this->objectUrl($this->value());
+        if(!is_null($this->value()))
+            return $this->objectUrl($this->value());
     }
 
     /**

Original file line number	Diff line number	Diff line change
`@@ -28,7 +28,8 @@ class Crop extends File`
`28`	`28`
`29`	`29`	`protected function preview()`
`30`	`30`	`{`
`31`		`- return $this->objectUrl($this->value());`
	`31`	`+ if(!is_null($this->value()))`
	`32`	`+ return $this->objectUrl($this->value());`
`32`	`33`	`}`
`33`	`34`
`34`	`35`	`/**`