fix server-side crashes

src/main/java/net/fretux/skillengine/api/SkillEngineAPI.java

@@ -3,7 +3,6 @@ package net.fretux.skillengine.api;
 import net.fretux.skillengine.SkillEngine;
 import net.fretux.skillengine.capability.PlayerSkillData;
 import net.fretux.skillengine.capability.SkillEngineCapabilities;
-import net.fretux.skillengine.client.SkilltreeClientState;
 import net.fretux.skillengine.skilltree.AbilityNode;
 import net.fretux.skillengine.skilltree.AbilityNodeRegistry;
 import net.fretux.skillengine.skilltree.SkillLogic;
@@ -11,6 +10,8 @@ import net.fretux.skillengine.skilltree.SkillNode;
 import net.fretux.skillengine.skilltree.SkillNodeRegistry;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.world.entity.player.Player;
+import net.minecraftforge.api.distmarker.Dist;
+import net.minecraftforge.fml.loading.FMLEnvironment;
 
 import java.util.List;
 import java.util.Set;
@@ -130,14 +131,28 @@ public final class SkillEngineAPI {
     }
 
     public static boolean clientHasSkill(ResourceLocation id) {
-        return SkilltreeClientState.isUnlocked(id);
+        return FMLEnvironment.dist == Dist.CLIENT && ClientAccess.clientHasSkill(id);
     }
 
     public static int clientGetAbilitySlot(ResourceLocation id) {
-        return SkilltreeClientState.getSlotOfAbility(id);
+        return FMLEnvironment.dist == Dist.CLIENT ? ClientAccess.clientGetAbilitySlot(id) : -1;
     }
 
     public static ResourceLocation[] clientGetAbilitySlots() {
-        return SkilltreeClientState.getAbilitySlots();
+        return FMLEnvironment.dist == Dist.CLIENT ? ClientAccess.clientGetAbilitySlots() : new ResourceLocation[3];
     }
-}
+
+    private static final class ClientAccess {
+        private static boolean clientHasSkill(ResourceLocation id) {
+            return net.fretux.skillengine.client.SkilltreeClientState.isUnlocked(id);
+        }
+
+        private static int clientGetAbilitySlot(ResourceLocation id) {
+            return net.fretux.skillengine.client.SkilltreeClientState.getSlotOfAbility(id);
+        }
+
+        private static ResourceLocation[] clientGetAbilitySlots() {
+            return net.fretux.skillengine.client.SkilltreeClientState.getAbilitySlots();
+        }
+    }
+}