Fix crash upon opening print preview with invalid printer.

chrome/browser/ui/webui/print_preview/printer_capabilities.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/webui/print_preview/printer_capabilities.h"
 
 #include <memory>
 #include <string>
 #include <utility>
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/strings/string_piece.h"
 #include "base/values.h"
 #include "chrome/common/cloud_print/cloud_print_cdd_conversion.h"
 #include "chrome/common/crash_keys.h"
 #include "content/public/browser/browser_thread.h"
 #include "printing/backend/print_backend.h"
 #include "printing/backend/print_backend_consts.h"
 
 namespace printing {
 
 const char kPrinterId[] = "printerId";
 const char kPrinterCapabilities[] = "capabilities";
 
 namespace {
 
-// Returns a Dictionary representing printer capabilities as CDD.
+// Returns a dictionary representing printer capabilities as CDD.  Returns
+// nullptr if a dictionary could not be generated.
 std::unique_ptr<base::DictionaryValue>
 GetPrinterCapabilitiesOnBlockingPoolThread(const std::string& device_name) {
   DCHECK(content::BrowserThread::GetBlockingPool()->RunsTasksOnCurrentThread());
   DCHECK(!device_name.empty());
 
   scoped_refptr<PrintBackend> print_backend(
       PrintBackend::CreateInstance(nullptr));
 
   VLOG(1) << "Get printer capabilities start for " << device_name;
   crash_keys::ScopedPrinterInfo crash_key(
@@ skipping 48 matching lines @@
   const auto printer_name_description =
       GetPrinterNameAndDescription(basic_info);
   const std::string& printer_name = printer_name_description.first;
   const std::string& printer_description = printer_name_description.second;
 
   auto printer_info = base::MakeUnique<base::DictionaryValue>();
   printer_info->SetString(kPrinterId, device_name);
   printer_info->SetString(kSettingPrinterName, printer_name);
   printer_info->SetString(kSettingPrinterDescription, printer_description);
 
-  printer_info->Set(kPrinterCapabilities,
-                    GetPrinterCapabilitiesOnBlockingPoolThread(device_name));
+  std::unique_ptr<base::DictionaryValue> capabilities =
+      GetPrinterCapabilitiesOnBlockingPoolThread(device_name);
+  if (capabilities)
+    printer_info->Set(kPrinterCapabilities, std::move(capabilities));
+
   return printer_info;
 }
 
 }  // namespace printing