fix multiplayer crash

app/src/main/java/gui/widget/SudokuRenderer.java

@@ -71,6 +71,9 @@ public class SudokuRenderer {
 
 	private void renderPopup() {
 		if (ImGui.beginPopup("editPopup")) {
+			if (currentCell == null)
+				ImGui.closeCurrentPopup();
+			else {
 				Block block = currentCell.getBlock();
 				int symbolCount = block.getCells().size();
 				for (int i = 0; i < symbolCount; i++) {
@@ -92,6 +95,7 @@ public class SudokuRenderer {
 						}
 					}
 				}
+			}
 			ImGui.endPopup();
 		}
 	}