FIX:fixed the crash caused by file transfer

jira:[none]

Change-Id: Ice1461eff200219792e4fb8b20aa36c0c176cd6f
(cherry picked from commit 38e784dd495d7baa3f1cf0cabde1f36be58693be)

src/slic3r/GUI/Printer/PrinterFileSystem.cpp

@@ -1098,6 +1098,10 @@ boost::uint32_t PrinterFileSystem::RequestMediaAbility(int api_version)
 
 void PrinterFileSystem::RequestUploadFile()
 {
+    if (!m_upload_file) {
+        return;
+    }
+
     json req;
     req["type"]    = "model";
     req["storage"] = m_upload_file->select_storage;