HistoryManager: make sure we have a waveform before attempting to free it

Author: azonenberg

src/ngscopeclient/HistoryManager.cpp

@@ -2,7 +2,7 @@
 *                                                                                                                      *
 * ngscopeclient                                                                                                        *
 *                                                                                                                      *
-* Copyright (c) 2012-2025 Andrew D. Zonenberg and contributors                                                         *
+* Copyright (c) 2012-2026 Andrew D. Zonenberg and contributors                                                         *
 * All rights reserved.                                                                                                 *
 *                                                                                                                      *
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the     *
@@ -419,6 +419,9 @@ bool HistoryManager::OnMemoryPressure(
 			auto& hist = it.second;
 			for(auto jt : hist)
 			{
+				if(jt.second == nullptr)
+					continue;
+
 				if(jt.second->HasGpuBuffer())
 				{
 					memFreed = true;