Use a default schema provider without path in case none is supplied

Even though no search path is provided we need to decode the URL and see
if we can find the schema in the current workspace.

Also fixes an issue where schema can't be opened by double click on
linux.

Fix #890

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/schema/SchemaInclude.java

@@ -14,6 +14,7 @@
 package org.eclipse.pde.internal.core.schema;
 
 import java.io.PrintWriter;
+import java.util.Objects;
 
 import org.eclipse.core.runtime.CoreException;
 import org.eclipse.pde.internal.core.PDECore;
@@ -24,6 +25,8 @@
 
 public class SchemaInclude extends SchemaObject implements ISchemaInclude {
 
+	private static final PathSchemaProvider DEFAULT_SCHEMA_PROVIDER = new PathSchemaProvider(null);
+
 	private static final long serialVersionUID = 1L;
 
 	private String fLocation;
@@ -57,7 +60,7 @@ public SchemaInclude(ISchemaObject parent, String location, boolean abbreviated,
 		super(parent, location);
 		fLocation = location;
 		fAbbreviated = abbreviated;
-		this.schemaProvider = schemaProvider;
+		this.schemaProvider = Objects.requireNonNullElse(schemaProvider, DEFAULT_SCHEMA_PROVIDER);
 	}
 
 	/**
@@ -98,7 +101,7 @@ public ISchema getIncludedSchema() {
 		if (fAbbreviated) {
 			SchemaRegistry registry = PDECore.getDefault().getSchemaRegistry();
 			fIncludedSchema = registry.getIncludedSchema(descriptor, fLocation);
-		} else if (fIncludedSchema == null && schemaProvider != null) {
+		} else if (fIncludedSchema == null) {
 			fIncludedSchema = schemaProvider.createSchema(descriptor, fLocation);
 		}
 		return fIncludedSchema;

ui/org.eclipse.pde.ui/src/org/eclipse/pde/internal/ui/editor/schema/SchemaEditor.java

@@ -187,10 +187,17 @@ public static boolean openSchema(IFile file) {
 
 	public static boolean openSchema(IPath path) {
 		String pluginId = path.segment(0);
+		int remove;
+		if ("schema:".equals(pluginId)) { //$NON-NLS-1$
+			pluginId = path.segment(1);
+			remove = 2;
+		} else {
+			remove = 1;
+		}
 		IPluginModelBase model = PluginRegistry.findModel(pluginId);
 		if (model != null && model.getUnderlyingResource() != null) {
 			IProject project = model.getUnderlyingResource().getProject();
-			IFile file = project.getFile(path.removeFirstSegments(1));
+			IFile file = project.getFile(path.removeFirstSegments(remove));
 			return openSchema(file);
 		}
 		return false;