增加错误保护

app/src/main/cpp/PhoneDevice.cpp

@@ -2171,6 +2171,10 @@ void visualize(const char* filename, const ncnn::Mat& m)
 
 void DrawOutlineText(cv::Ptr<cv::ft::FreeType2> ft2, cv::Mat& mat, const std::string& str, cv::Point startPoint, int fontSize, cv::Scalar clr, int thickness)
 {
+    if (mat.empty())
+    {
+        return;
+    }
     std::vector<std::string> lines = split(str, "\n");
     int lineHeight = 0;
     cv::Point pt = startPoint;