ImGui: couple fixes for different cause of crashes

src/core/common/video/RenderBase.hpp

@@ -31,7 +31,9 @@ public:
 	}
 
 	void DeviceRelease() {
-		m_device_release();
+		if (m_device_release) {
+			m_device_release();
+		}
 	}
 
 	void SetWindowRelease(const std::function<void()>& func_register) {
@@ -39,7 +41,9 @@ public:
 	}
 
 	void WindowRelease() {
-		m_window_release();
+		if (m_window_release) {
+			m_window_release();
+		}
 	}
 
 protected:

src/core/kernel/init/CxbxKrnl.cpp

@@ -1783,9 +1783,10 @@ void CxbxKrnlShutDown(bool is_reboot)
 	g_VMManager.Shutdown();
 
 	// Shutdown the render manager
-	g_renderbase->Shutdown();
-	g_renderbase.release();
-	g_renderbase = nullptr;
+	if (g_renderbase != nullptr) {
+		g_renderbase->Shutdown();
+		g_renderbase = nullptr;
+	}
 
 	CxbxUnlockFilePath();