WebGPURenderer: Fix clear for default framebuffer

Author: RenaudRohlinger

src/renderers/common/Renderer.js

@@ -1911,7 +1911,11 @@ class Renderer {
 		}
 
 		// #30329
-		renderContext.clearColorValue = this._clearColor;
+		if ( renderContext !== null ) {
+
+			renderContext.clearColorValue = this._clearColor;
+
+		}
 
 		this.backend.clear( color, depth, stencil, renderContext );