Fixed the following NullReferenceException:

System.NullReferenceException: 
   in OpenHardwareMonitor.Hardware.Nvidia.NvidiaGroup..ctor()
   in OpenHardwareMonitor.Hardware.Computer.Open()
   in OpenHardwareMonitor.GUI.MainForm..ctor()
   in OpenHardwareMonitor.Program.Main()

Hardware/Nvidia/NvidiaGroup.cs

@@ -55,7 +55,13 @@ namespace OpenHardwareMonitor.Hardware.Nvidia {
 
       NvPhysicalGpuHandle[] handles = 
         new NvPhysicalGpuHandle[NVAPI.MAX_PHYSICAL_GPUS];
-  
+
+      if (NVAPI.NvAPI_EnumPhysicalGPUs == null) {
+        report.AppendLine("Error: NvAPI_EnumPhysicalGPUs not available");
+        report.AppendLine();
+        return;
+      }
+
       int count;
       if (NVAPI.NvAPI_EnumPhysicalGPUs(handles, out count) != NvStatus.OK)
         return;