Fix rare null pointer race condition

core/src/main/java/com/seibel/distanthorizons/core/dataObjects/render/bufferBuilding/ColumnRenderBuffer.java

@@ -84,9 +84,11 @@ public class ColumnRenderBuffer implements AutoCloseable
 	/** Should be run on a DH thread. */
 	public synchronized CompletableFuture<ColumnRenderBuffer> makeAndUploadBuffersAsync(LodQuadBuilder builder, EDhApiGpuUploadMethod gpuUploadMethod)
 	{
-		if (this.uploadFuture != null)
+		// separate variable to prevent race condition when checking null
+		CompletableFuture<ColumnRenderBuffer> future = this.uploadFuture;
+		if (future != null)
 		{
-			return this.uploadFuture;
+			return future;
 		}
 		this.uploadFuture = new CompletableFuture<>();
 		

core/src/main/java/com/seibel/distanthorizons/core/dataObjects/render/bufferBuilding/ColumnRenderBufferBuilder.java

@@ -73,7 +73,7 @@ public class ColumnRenderBufferBuilder
 		uploadFuture.whenComplete((uploadedBuffer, exception) -> 
 		{
 			// clean up if not uploaded
-			if (!uploadedBuffer.buffersUploaded)
+			if (uploadedBuffer != null && !uploadedBuffer.buffersUploaded)
 			{
 				uploadedBuffer.close();
 			}