Fix hard crash when terrain is missing from map

Previously, opening a map that referenced a deleted or missing terrain
caused a severe crash with multiple error dialogs, forcing users to kill
the process. This change restores the previous behavior where missing
terrains are gracefully handled: a warning is logged and the terrain is
substituted with a fallback placeholder (ErrorTexture from
TextureManager).

This significantly improves UX for developers and modders working with
outdated or broken maps, aligning behavior with expectations from
earlier versions of the editor.

source/graphics/TerrainTextureEntry.cpp

@@ -137,8 +137,8 @@ CTerrainTextureEntry::CTerrainTextureEntry(CTerrainPropertiesPtr properties, con
 	if (CRenderer::IsInitialised())
 		m_TerrainAlpha = g_TexMan.LoadAlphaMap(alphamap);
 
-	float texAngle = 0.f;
-	float texSize = 1.f;
+	float texAngle{0.f};
+	float texSize{1.f};
 
 	if (m_pProperties)
 	{
@@ -146,19 +146,33 @@ CTerrainTextureEntry::CTerrainTextureEntry(CTerrainPropertiesPtr properties, con
 		texAngle = m_pProperties->GetTextureAngle();
 		texSize = m_pProperties->GetTextureSize();
 	}
-
-	m_TextureMatrix.SetZero();
-	m_TextureMatrix._11 = cosf(texAngle) / texSize;
-	m_TextureMatrix._13 = -sinf(texAngle) / texSize;
-	m_TextureMatrix._21 = -sinf(texAngle) / texSize;
-	m_TextureMatrix._23 = -cosf(texAngle) / texSize;
-	m_TextureMatrix._44 = 1.f;
+	GenerateTextureMatrix(texAngle, texSize);
 
 	GroupVector::iterator it=m_Groups.begin();
 	for (;it!=m_Groups.end();++it)
 		(*it)->AddTerrain(this);
 }
 
+CTerrainTextureEntry::CTerrainTextureEntry(const CStr tag):
+	m_pProperties(nullptr),
+	m_BaseColor(0),
+	m_BaseColorValid(false),
+	m_DiffuseTexturePath(""),
+	m_Tag(tag)
+{
+	if (!CRenderer::IsInitialised())
+		return;
+
+	const VfsPath alphamap{"standard"};
+	const VfsPath mat{VfsPath{"art/materials"} / "terrain_norm_spec.xml"};
+	m_Material = g_Renderer.GetSceneRenderer().GetMaterialManager().LoadMaterial(mat);
+	const CTexturePtr texptr{g_Renderer.GetTextureManager().GetErrorTexture()};
+	m_Material.AddSampler(CMaterial::TextureSampler{str_baseTex, texptr});
+	m_TerrainAlpha = g_TexMan.LoadAlphaMap(alphamap);
+
+	GenerateTextureMatrix(0.0f, 1.f);
+}
+
 CTerrainTextureEntry::~CTerrainTextureEntry()
 {
 	for (GroupVector::iterator it=m_Groups.begin();it!=m_Groups.end();++it)
@@ -184,3 +198,13 @@ void CTerrainTextureEntry::BuildBaseColor()
 		m_BaseColorValid = true;
 	}
 }
+
+void CTerrainTextureEntry::GenerateTextureMatrix(const float texAngle, const float texSize)
+{
+	m_TextureMatrix.SetZero();
+	m_TextureMatrix._11 = cosf(texAngle) / texSize;
+	m_TextureMatrix._13 = -sinf(texAngle) / texSize;
+	m_TextureMatrix._21 = -sinf(texAngle) / texSize;
+	m_TextureMatrix._23 = -cosf(texAngle) / texSize;
+	m_TextureMatrix._44 = 1.f;
+}

source/graphics/TerrainTextureEntry.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2022 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -37,6 +37,19 @@ public:
 	// Most of the texture's data is delay-loaded, so after the constructor has
 	// been called, the texture entry is ready to be used.
 	CTerrainTextureEntry(CTerrainPropertiesPtr props, const VfsPath& path);
+
+	/**
+	 * @brief Constructs a terrain texture entry with the given tag.
+	 *
+	 * This constructor creates a placeholder (dummy) texture entry,
+	 * allowing the system to handle missing terrain textures gracefully.
+	 * It ensures stability by avoiding crashes when a texture file is
+	 * not found.
+	 *
+	 * @param tag The identifier for the terrain texture entry.
+	 */
+	CTerrainTextureEntry(const CStr tag);
+
 	~CTerrainTextureEntry();
 
 	const CStr& GetTag() const { return m_Tag; }
@@ -67,6 +80,7 @@ public:
 	CTerrainTextureManager::TerrainAlphaMap::iterator m_TerrainAlpha;
 
 private:
+	void GenerateTextureMatrix(const float texAngle, const float texSize);
 	// Tag = file name stripped of path and extension (grass_dark_1)
 	CStr m_Tag;
 

source/graphics/TerrainTextureManager.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -67,7 +67,7 @@ void CTerrainTextureManager::UnloadTerrainTextures()
 	m_LastGroupIndex = 0;
 }
 
-CTerrainTextureEntry* CTerrainTextureManager::FindTexture(const CStr& tag_) const
+CTerrainTextureEntry* CTerrainTextureManager::FindTexture(const CStr& tag_)
 {
 	CStr tag = tag_.BeforeLast("."); // Strip extension
 
@@ -75,8 +75,13 @@ CTerrainTextureEntry* CTerrainTextureManager::FindTexture(const CStr& tag_) cons
 		if (te->GetTag() == tag)
 			return te;
 
-	LOGWARNING("CTerrainTextureManager: Couldn't find terrain %s", tag.c_str());
-	return 0;
+	LOGWARNING("CTerrainTextureManager: Couldn't find terrain %s using fallback texture", tag.c_str());
+
+	// If the texture is not found, return a default texture.
+	// This is a fallback texture, so it should not be used in the editor.
+	CTerrainTextureEntry* fallback{new CTerrainTextureEntry{tag}};
+	m_TextureEntries.push_back(fallback);
+	return fallback;
 }
 
 CTerrainTextureEntry* CTerrainTextureManager::AddTexture(const CTerrainPropertiesPtr& props, const VfsPath& path)

source/graphics/TerrainTextureManager.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2023 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -103,7 +103,7 @@ public:
 
 	void UnloadTerrainTextures();
 
-	CTerrainTextureEntry* FindTexture(const CStr& tag) const;
+	CTerrainTextureEntry* FindTexture(const CStr& tag);
 
 	// Create a texture object for a new terrain texture at path, using the
 	// property sheet props.